Association football has multiple statuses/types of matches.
A definitive list with such available data of significant attendance figures for every match would be impossible to produce; The list therefore includes senior matches of: In most cases, some attendance figures for a specific match may be different to other sources or claims (e.g. Uruguay v Brazil in the 1950 FIFA World Cup where the official figure is 173,850, but is also claimed as approximately 200,000).
[3] Uncertainties around decisions on which figures to include is a major difficulty when recording highest attendances in association football.
Therefore; sources of attendance figures include those that are claimed by official reports of match organisers (e.g. FIFA for FIFA tournaments, confederations for their respective tournaments, football associations for their respective competitions, etc.)
If such data not available by organisers, next sources are International Federation of Football History & Statistics (IFFHS), then the closest relevant source of the figure.