A message segment is the unit US carriers and providers bill RCS Rich text by: 160 bytes of UTF-8 text. A message of 300 bytes is billed as two segments.
Segments are for billing only
The customer sees one message, not several pieces; the segment count is a calculation on the bill. The words on reply buttons are not counted. Rich Media messages are not billed by segment at all: they are one flat charge whatever their length.
Why bytes, not characters
Plain letters and numbers take one byte each, but emoji and accented letters take two to four. A 140-character message with a few emoji can pass 160 bytes and bill as two segments, so short, plain wording is cheaper. On SimplyRCS each segment is $0.0039; see How RCS billing works.