What are "server_version_split" and "number_of_event_types" in "FormatDescriptionEvent" class?

code reference:

python-mysql-replication/pymysqlreplication/event.py

Line 397 in ea61cca

self.server_version_split = struct.unpack("<3B", self.packet.read(3))

python-mysql-replication/pymysqlreplication/event.py

Line 398 in ea61cca

self.number_of_event_types = struct.unpack("<B", self.packet.read(1))[0]

There are two attributes, "server_version_split" and "number_of_event_types", and i have not find those two fields in MySQL documents (I saw https://dev.mysql.com/doc/dev/mysql-server/8.0.40/page_protocol_replication_binlog_event.html).

So what are they? Maybe they are in some older MySQL version?

Thanks.