So far from most of the sheet scores I've read, a2 is used mostly in wood and brass instrument staves, while unis. (and on rarer occasion, tutti) is in string instruments. It is even possible to see all three in a single score.
Is there any common practice on when they should be used? How to differentiate between them? I have seen some discussion but not quite sure if it's really the standard.
The reason for the discrepancy is that there are generally far fewer winds and brass in an orchestra, and so the individual parts are usually numbered. Thus, you might see a part labeled with individual numbers (such as "1. 2." for different flute or oboe parts, for instance, or a marking such as "a 2" to indicate that they should play together.
Since for the most part such divisions do not happen in string parts (although there are some exceptions), it would not make sense to say "a 2" or "a 7" or whatever other number of strings are involved. Instead, they should play together, and for this purpose, "unis." can be used. On the other hand, "tutti" is used to end a passage previously marked for one or more soloists, usually for string or choral parts.
It might be useful to think of it this way.
The second violins, for instance, are a section; they normally play together as a section. Many scores don't specify how many second violins are in the orchestra; they just expect there to be as many as the orchestra has.
When the composer needs this second violin section to change its behavior and split into two parts, the composer uses the label "divisi." This indicates that these two notes aren't being played as a double stop by each musician; the different notes are being played by different musicians.
"unison" is used to return to the "default" behavior of a string section: playing together as a section.
On the other hand, traditionally, orchestral scores listed exactly how many woodwinds would be playing. The first and second flutes are two separate "sections," and the expected default behavior is that they would be playing separate notes. Each musician has his or her own part, that shows only his or her own notes on it.
In the orchestral score, to save space, these two parts are sometimes combined. "a2" is how we indicate that we are changing this default behavior; it lets the conductor know that the same notes are appearing in both flute parts.
"Tutti" is used for a different purpose though. Unison markings distinguish passages from those that are divided; "tutti" passages distinguish those from passages played solo or by a small ensemble. As 'loco' is employed to restore notation to normal after an 8va/8vb marking, unison is used to indicate the end of any divisi, whereas 'tutti' is used to indicate the end of any solo or small ensemble passage and where the full complement of players (or singers) is to chime back in.
'a2' also shows up in percussion parts. even when there is only one written part (snare drum, for ex.) a composer occasionally wants a second player to 'double' what is being played, either for volume or added color/texture. bolero is a good example. toward the end, a 2nd snare drummer is supposed to join in. (this is not always done, but it is in the score/part.) i think shostakovich also does this in symphony no.7. berlioz also does it on a tambourine part. (etc...)
