unitTags normally are tags like "player" (yourself) or "group1", "group2", ... "group".. index for the group members (maybe that companions count as group .. n too but not sure anymore if they got a dedicated tag)
GetAbilityName does not need any unitTag though, even ZOs code does not use any, only the abilityId (1 param).
But many other GetABility* functions use the casterUnitTag now, e.g.
function GetAbilityCastInfo(abilityId, overrideRank, casterUnitTag) end
According to Zos code it can be e.g. "player", example here:
Code:
function ZO_Tooltip:AddAbilityStats(abilityId, overrideActiveRank, overrideCasterUnitTag)
overrideCasterUnitTag = overrideCasterUnitTag or "player"