These differ from tactics because they are more broad in perspective, anyways, enjoy.
Counter-Sniping: this is when a sniper actively hunts and eliminates enemy snipers, this is very hard, unless the enemy sniper is moving and you are lying in ambush. The reason for it being difficult is that normally the other sniper will have the advantage of already being in place, wheras you must set up (and possibly be detected)
Long-Range Support: this is when you are assigned to a squad and take out threats that are well concealed or in a easily defended area, in this role you should travel light (possibly leaving your ghillie behind if you have one) as you will need to keep up with the rest of the guys
Flanking: same as above but it is your duty to attack from behind or the sides to allow "your" squad to storm the enemies bunkers more easily
Scouting: this is where you go to a place with a good field of view and report enemy movements over a radio, firing only when fired upon or when discovered
Assasination: when you must sneak into the enemy base and assasinate their "General", normally this is a "one way" type mission as the bases are heavily guarded this could also apply to squad leaders