:conditional seems to be a key suffix you can attach to existing key to either change the default value or in some cases just show at what condition it's 'yes'. For example 
parking:maxstay:conditional=xxx or access:conditional=xxx

"parking" key is used to describe parking types, so while  parking:conditional is possible, it could be used to describe a change from "on_kerb" parking to "lane" parking at certain conditions, for example.