The access=designated wiki page explains the concept very clearly: " *NOTE!* The exact key/value combination access <https://wiki.openstreetmap.org/wiki/Key:access>=designated should *never* appear on an object. The value * <https://wiki.openstreetmap.org/wiki/Key:*> =designated must be used with a specific mode of transport. Examples: bicycle <https://wiki.openstreetmap.org/wiki/Key:bicycle>=designated <https://wiki.openstreetmap.org/wiki/Tag:bicycle%3Ddesignated> or foot <https://wiki.openstreetmap.org/wiki/Key:foot>=designated <https://wiki.openstreetmap.org/wiki/Tag:foot%3Ddesignated>. "
You could make a case for using bus=designated plus bicycle=designated for bus-and-bicycle-only roads. This would be in line with the tagging of foot-cycle-paths with bicycle=designated plus foot=designated
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