When I am using stealth ability, is it impossible to detect me unless I gone to the near of the enemy or is it just hard to see me?

It's just harder to see you. Abilities that 'increase your effective stealth level by x' make it harder to see you.

 

Some large packs of enemies and many bosses in this game have passive stealth detection; if you stealth within x meters of that pack, they'll detect you and that'll start the fight. Even if you could have been standing near them out of stealth without any trouble.

 

Some enemies in this game, especially droids, cast stealth scans.

In stealth, you are completely invisible to enemies and cannot be targeted by them, unless you get t0o close to an enemy or get decloaked (by receiving area damage, for example).

 

The distance you are allowed to get close before enemies can see you depends on your stealth level versus their detection level. But "on average" you should get in to melee range of most enemies/Players of your level before they can see you.

Edited February 14, 2014 by Arutar