The fixed part of the event data goes under different names, depending on which source file, work log, bug report, etc. you are reading:
Sometimes it is called the "fixed data" part, as in this discussion.
Sometimes it is called the "post-headers" part.
To make things notationally interesting, sometimes the fixed data part is referred to as the "event-specific headers" part of the event. That is, the word "header" is used in reference to a portion of the data part. One manifestation of this notational phenomenon appears in
log_event.h, where you will find the symbol
LOG_EVENT_MINIMAL_HEADER_LENdefined as 19 (the header length for v3 and v4), plus other symbols with names of the form
XXX_HEADER_LENfor different event types. The former symbol is the size of the event header (always 19). The latter symbols define the size of the fixed portion of the data part that is to be treated as the event-specific headers. For example,
ROTATE_HEADER_LENis 8 because a
ROTATE_EVENThas an 8-byte field in the fixed data part that indicates the position in the next log file of the first event in that file.
The variable part of event data also goes under different names, such as the event "payload" or "body."