Stealth assumes technology involved in the build and hull of the ship, not ECM type gadgets. For the latter, see ELINT. Technology such as Minbari jammers is considered to be ELINT.
Stealth adds to the effective range of the ship for purposes of being detected or hit. Stealth takes up mass, due to shape and material requirements being chosen for stealth rather than volume efficiency.
Each 5% of volume for stealth adds 3” to effective range of the ship for purposes of being detected or fired at. However, the effective range can never be more than double the actual range. A highly cloaked ship with 50% stealth, has a +30” modifier, but if they are only 6” from a target, they still only count as being effectively 12”.