And one could ask are segments without well data segment a segments? As mostly 2 first PDUs in exchange do not contain data...

Yes they are. I don't know what "well data segment" is, but perhaps you mean a payload. TCP streams consist of segments, and the fragmentation or collection of those logical segments into packets is a function of the lower-layer protocols which are not TCP.

https://www.rfc-editor.org/rfc/rfc9293.html

I think one important mention here is that fragmentation happens at two levels: most commonly, an upper layer protocol message (such as an HTTP request) is fragmented into several TCP segments, each with one TCP header. Those TCP segments themselves can sometimes be fragmented into multiple IP packets if the TCP MSS (maximum segment size) is larger than the Ethernet MTU (maximum transmission unit).

IP fragmentation is typically avoided, and in IPv6 it is only allowed at the originator of a packet - if a router is trying to route an IPv6 packet larger than the MTU of the outgoing link's MTU, it must drop the packet and send a signal about the MTU back. An IPv4 router may fragment the packet, but this rarely works well in practice.

I suppose, if you want to squeeze it into the definition given in RFC 9293, but that same RFC only uses "fragmentation" to denote "internet fragments" or IP-level fragmentation.

You're referring to "segmentation" which is also defined in that RFC. https://www.rfc-editor.org/rfc/rfc9293.html#name-segmentatio...

Appreciate pedantry when it’s absolutely warranted. Well said!