The RERA carpet INCLUDES the internal wall areas and EXCLUDES (1) all external walls, (2) all balconies, (3) dry spaces, (4) flower beds (5) all common areas.
Whereas the USABLE CARPET AREA excludes all internal walls but includes (1) balconies (2) dry spaces (3) niche areas (4) flower beds and (5) all projections inside the livable area that one can use.
Hence USABLE CARPET AREA is always more than the RERA CARPET AREA. Also, one should note that at times developers or builders consider internal walls area occupied in USABLE CARPET areas.