Three terms describe distinct stopping techniques. With iskan, the last vowel is wholly removed. With rawm, a very small part of a kasrah or dammah is audible. With ishmam, the dammah is suppressed and represented only by rounded lips.
Iskan: the consonant ends in sukun; no final vowel is voiced.
Rawm: a faint sound; a nearby listener may hear the small remainder of the vowel.
Ishmam: a silent sign; someone watching may see the lip rounding.
These are three different technical acts, not interchangeable names. A reciter must use a form only where permitted and must avoid turning rawm into a full vowel or ishmam into an audible nasal or rounded vowel sound.