Target is a special tag that gives you the target of the current unit, so [Target(unit='mouseover')] => "mouseovertarget", since unit is 'mouseover' by default in your case, [Target] => "mouseovertarget"
This isn't the best way to do it, instead, go with
I've been looking for a dogtag sequence that would modify the class text so it would show the class only if target is a player and creature race only if target is an NPC/mob. The first part is easy with "playerclass" but I can't get the latter part to work. It keeps showing the race for players as well as mobs.
Alright, I added a semi-important feature to DogTag: internal expressions.
It is possible to embed DogTag-1.0 sequences within other sequences. e.g. "[Text([Level] [Class])]" is functionally equivalent to "[Level] [Class]". This can be useful in such occasions such as "[Level:IsLess(Level#player)]". It is also possible to embed sequences within the core sequence itself, e.g. "[[Status:CurHP]:Green]" will highlight the result of "[Status:CurHP]" green, whereas "[Status:CurHP:Green]" would technically only highlight the result of "[CurHP]"
I'd like to see what people can make with this.
Note: don't be afraid to use this over other mechanisms, it's quite cheap.