The rawī is the central letter upon which an Arabic poetic rhyme is founded. The technical qāfiyah is broader: it can include the rawī and other surrounding metrical or phonetic elements.
For luzūm mā lā yalzam, the main issue is whether an extra feature before the rawī has been consistently preserved. One need not confuse that added condition with the basic requirement of rhyme itself.
Two verses may share the same rawī even when the preceding consonants differ. Voluntarily fixing those additional consonants creates a stricter verbal pattern.