Hi,
sometimes contributor-photographer to such services are asked to provide a spreadsheet or database file which contains for each photo the recognizable numbers of pictured athletes. This information can be linked to eventual official participant lists of such events as well as being used by online photo services in search functions based on known numbers. Customers who don't know the numbers generally have to look through lists of photos do find the wanted one. Online photo agencies use keywords for searching which have to be provided by the photographer. Keywording occurs mostly by filling out the so called IPTC fields which is data directly stored together with the image data as is the EXIF data which contains information about the camera used and details of the exposure.
Hope this answers your question.
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Cheers,
Michael Fritzen