Bolter weapons, bolt pistols and boltguns are standard, everyone stats with them, some people remain with them no matter what.
Storm bolters, flamer and melta weapons are more specialised, requiring specialist training. Sisters that use them are called "Dominions", though Dominion squads contain sisters only trained for normal bolters.
Heavy weapons, like heavy flamers, heavy bolters and multimeltas are likewise used by specialists. Heavy weapon squads are called Retributors, I don't know if that name is used of the heavy weapon specialists.
Melta pistols (infernus pistols) are extremely rare and valuable, generally only used by the most high ranking and important people. Plasma pistols likewise, but I think they are less rare.
Flamer pistols (hand flamers) seem to be only used by members of Seraphim squads.
Seraphim squads are elite, jump-pack equipped Sisters that are trained to fight with a pistol in each hand (not as stupid as it might seem, using a jump-pack might stop you from using a long arm). They use bolt pistols as standard, some use a pair of hand flamers or infernus pistols.
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Power weapons are used only by higher ranking sisters. You also get the odd fancy blessed weapon, but they are very rare, and each of them should be a unique, one off thing probably, with it's own history, not like a power sword which can almost be mass produced.
