How is 10GBASE-T defined for providing compatibility?

10GBASE-T isn't downward compatible (nor is any other Ethernet physical-layer protocol by itself downward compatible to anything). A port labeled "10GBASE-T" may be downward compatible.

Twisted-pair Ethernet uses Autonegotiation where each side announces its capabilities (speed, duplex and line code) using Fast Link Pulses (FLP), so the link partners can agree on the best mutually supported protocol before PHY initialization begins.

It's up to a port's capabilities and configuration which protocols it supports and announces. Many 10GBASE-T ports support 1000BASE-T as well, some even 100BASE-TX or 10BASE-T.

The Autonegotiation process is detailed in IEEE 802.3 Clause 28. The protocol priority is selected from the list in Annex 28B, higher is better:

a) 40GBASE-T full duplex
b) 25GBASE-T full duplex
c) 10GBASE-T full duplex
d) 5GBASE-T full duplex
e) 2.5GBASE-T full duplex
f) 1000BASE-T full duplex
g) 1000BASE-T (half duplex - obsolete)
h) 100BASE-T2 full duplex (obsolete)
i) 100BASE-TX full duplex
j) 100BASE-T2 (obsolete)
k) 100BASE-T4 (obsolete)
l) 100BASE-TX (half duplex)
m) 10BASE-T full duplex
n) 10BASE-T (half duplex)