Manual:$wgInterwikiCache

Details
1>Special:MyLanguage/Interwiki cache|Interwiki caching consists of the following four settings:

This setting specifies an array in the following format, or a string with the path to  file containing the same keys. This format can be generated with .
 * dbname:key -

a simple key (e.g. enwiki:meta)
 * _sitename:key -

site-scope key (e.g. wiktionary:meta)
 * __global:key -

global-scope key (e.g. __global:meta)
 * __sites:dbname -

site mapping (e.g. __sites:enwiki)

Sites mapping just specifies site name, other keys provide "local url" data layout.

This setting specifies the number of domains to check for messages:

Just wiki(db)-level

wiki and global levels

site levels

This setting is used if unable to resolve from the cache.

Since  it is possible to get the list of cached interwiki prefixes, used in the siteinfo>Special:MyLanguage/API:Siteinfo|API and  extension.