Here is a list of all documented variables with links to the struct/union documentation for each field:

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a : Blitter_32bppSimple::Pixel, ColourBGRA, ColourRGBA, DiagonalTileArea, SpriteLoader::CommonPixel
a_cur : DiagonalTileIterator
a_max : DiagonalTileIterator
abbrev : CargoSpec
abort : GenWorldInfo
abortp : GenWorldInfo
absolute : NWidgetResizeBase
abstract : FiosType
abstract_filetype : ConsoleFileList
acc_duration : anonymous_namespace{framerate_gui.cpp}::PerformanceData
acc_timestamp : anonymous_namespace{framerate_gui.cpp}::PerformanceData
acceleration : ShipVehicleInfo, Vehicle
acceleration_type : RailTypeInfo
acceptance : IndustryTileSpec, NewsSettings
accepted : Industry
accepted_cargo_filter_criteria : IndustryDirectoryWindow
accepts_cargo : HouseSpec, IndustrySpec, IndustryTileSpec
accepts_cargo_label : HouseSpec, IndustrySpec, IndustryTileSpec
accepts_lines : StationViewWindow
ACCEPTS_LINES : StationViewWindow
accident : NewsSettings
accident_other : NewsSettings
accumulated_waiting : Industry::AcceptedCargo
accuracy : DemandCalculator, LinkGraphSettings
acs : NWidgetNewGRFDisplay
act5_type : SpriteAlignerWindow
action : SaveLoadParams
action14present : NewGRFParametersWindow
ACTION_CLEAR : QueryString
action_counts : VehicleCargoList
ACTION_DESELECT : QueryString
ACTION_NOTHING : QueryString
active : NewGRFProfiler
active_button_id : StoryBookWindow
active_over : NewGRFWindow
active_playlist : MusicSystem
active_sel : NewGRFWindow
actives : NewGRFWindow
add_accepted_cargo_proc : TileTypeProcs
add_output : IndustryProductionSpriteGroup
add_produced_cargo_proc : TileTypeProcs
address : NetworkAddress, ServerNetworkAdminSocketHandler
address_length : NetworkAddress
address_proc : SaveLoad
addresses : TCPConnecter
admin_authorized_keys : NetworkSettings
admin_name : NetworkAdminSocketHandler
admin_password : NetworkSettings
admin_version : NetworkAdminSocketHandler
advance : ICURun
advanced : SelectCompanyManagerFaceWindow
advanced_vehicle_list : GUISettings
advice : NewsSettings
advice_type : NewsItem
afilter : Textbuf
after : AlternatingIterator< Titer >
age : Engine, NewsTypeData, Vehicle
ai : GameSettings, NetworkCompanyStats, ScriptConfigSettings, TCPConnecter
ai_callback : GenericScopeResolver
ai_developer_tools : GUISettings
ai_disable_veh_aircraft : AISettings
ai_disable_veh_roadveh : AISettings
ai_disable_veh_ship : AISettings
ai_disable_veh_train : AISettings
ai_in_multiplayer : AISettings
ai_passenger_only : RailVehicleInfo
air_drag : RailVehicleInfo, RoadVehicleInfo
airport : CompanyInfrastructure, Station
airport_class : TileDesc
airport_id : AirportTileScopeResolver
airport_name : TileDesc
airport_scope : AirportTileResolverObject
airport_tile_name : TileDesc
align : NWidgetCore, NWidgetPart::NWidgetPartUnion, NWidgetPartAlignment
align_h : IntroGameViewportCommand
align_v : IntroGameViewportCommand
all_capacities : TestedEngineDetails
alloc_cache : Pool< Titem, Tindex, Tgrowth_step, Tpool_type, Tcache >
allocated_size : ScriptAllocator
allocation_limit : ScriptAllocator
allocator : Squirrel
allow_do_command : ScriptStorage
allow_insecure_admin_login : NetworkSettings
allow_list : CompanyProperties
allow_merge : LinkRefresher
allow_town_level_crossings : EconomySettings
allow_town_roads : EconomySettings
ALLOWED_DRIFT : VideoDriver
alternate_labels : RailTypeInfo, RoadTypeInfo
always_accepted : Station
ambient : SoundSettings
amount : CargoSummaryItem, OldChunks
amount_fract : GoodsEntry
amount_of_rivers : GameCreationSettings
anim_alloc : Blitter_32bppAnim
anim_buf : Blitter_32bppAnim
anim_buf_height : Blitter_32bppAnim
anim_buf_pitch : Blitter_32bppAnim
anim_buf_width : Blitter_32bppAnim
anim_buffer : OpenGLBackend, VideoDriver_CocoaOpenGL, VideoDriver_SDL_OpenGL
anim_next : IndustryTileSpec
anim_pbo : OpenGLBackend
anim_production : IndustryTileSpec
anim_state : IndustryTileSpec
anim_texture : OpenGLBackend
animate_cur : CursorVars
animate_list : CursorVars
animate_timeout : CursorVars
animation : AirportTileSpec, HouseSpec, IndustryTileSpec, ObjectSpec
animation_special_flags : AirportTileSpec
animation_state : EffectVehicle
animation_substate : EffectVehicle
api_name : Squirrel
api_version : AIInfo, GameInfo, ScriptInstance
appear_creation : IndustrySpec
appear_ingame : IndustrySpec
area : ClearedObjectArea, SelectStationWindow< T >
arguments : GetOptData
array : CargoSpec
arrival : TimetableArrivalDeparture
arrival_other : NewsSettings
arrival_player : NewsSettings
ascender : FontCache
aspect : NWidgetPart::NWidgetPartUnion
aspect_flags : NWidgetBase
aspect_ratio : NWidgetBase
async_mode : ScriptStorage
async_mode_instance : ScriptStorage
at : LoggedAction
att : AirportTileTableIterator
authentication_handler : ClientNetworkGameSocketHandler, ServerNetworkAdminSocketHandler, ServerNetworkGameSocketHandler
author : ScriptInfo
authorized_key_handler : X25519AuthorizedKeyServerHandler
authorized_keys : NetworkAuthenticationDefaultAuthorizedKeyHandler
auto_euro : GUISettings
auto_rail : RailTypeInfo
auto_refit : RefitWindow
auto_remove_signals : GUISettings
auto_road : RoadTypeInfo
auto_scrolling : GUISettings
auto_select : NetworkContentListWindow
autoclean_companies : NetworkSettings
autoclean_novehicles : NetworkSettings
autoclean_protected : NetworkSettings
autorail : RailTypeInfo
autoreplace_defined : GroupStatistics
autoreplace_finished : GroupStatistics
autoroad : RoadTypeInfo
autosave_interval : GUISettings
autosave_on_exit : GUISettings
autosave_on_network_disconnect : GUISettings
autoscroll : ScriptDebugWindow
autoslope : ConstructionSettings
avail_pos : NewGRFWindow
avail_railtypes : Company
avail_roadtypes : Company
avail_sel : NewGRFWindow
avail_year : BridgeSpec
available_actions : TownAuthorityWindow
available_sets : BaseMedia< Tbase_set >
avails : NewGRFWindow
avs : NWidgetNewGRFDisplay
awarded : Subsidy
axis : StationPickerSelection, StationScopeResolver