The Sibelius Sounds library which ships with Sibelius 7 and later does not have support for reduced player configurations such as a2, a3, a4 etc., but if you are a NotePerformer user, you are in luck.
NotePerformer 3 installs a plugin in Sibelius called “NotePerformer a2, a3 … MIDI message” which appears in the Plug-ins page of the home tab.
This plugin adds a MIDI controller message to a beat, or the start of a selected region for playback when using NotePerformer.
In other words, you can apply this plugin to add hidden MIDI controller info at the same location as the “a2” text, so that NotePerformer produces correct playback of a2 etc.
You can use the plugin to add this MIDI Controller data each time, or you can also add this controller info directly in your score either manually or automatically with NotePerformer.
more >> “Add a2, a3, a4 playback for NotePerformer in Sibelius”