A2 text

[k.k]

Trbn. 3staves—Enter the note in the first and third staves, but what does a2 mean? Is it just a matter of adding text?

[charles_piano]

It’s a condensed staff, and I.III a2 means that Trombone 1 and three are both playing the same notes.

[k.k]

Thanks.

So is that the instrument corresponding to a2?

[k_b]

If you notate that into your score, you will start with three individual trombone players/staves.
Input the notes from the first bar into the three trombones accordingly.
Input the first note from the fifth bar into trombone I and trombone III only.
Dorico will handle the condensation later automatically.

[charles_piano]

“a 2” means together when you speak about two players. “a 3” would mean three players together, etc.
Dorico handles this automatically:

[k.k]

The ‘a2’ text isn’t appearing—do I need to add it in the ‘Custom’ section?

[charles_piano]

Maybe check this option (Engraving Options>Condensing):

[k.k]

Thanks.