Is it

i) the most expensive fare on a route in a certain class
ii) a fare which uses the high booking class of that airline
iii) a fare listed as "XXX class unrestricted."
iv) All of the above?
v) something else?

Example:

Business Class fares between XXX and YYY.

1 is the cheapest, 4 is the most expensive.

Fare 1: CRT - $300 changes / non ref
Fare 2: JRT - no penalties but listed as "Business Restricted"
Fare 3 : J2 - no penalties listed as "unrestricted"
Fare 4: J - no penalties listed as "unrestricted"

In this case would 2 or 3 be "full fare" tickets?

Thanks!

Cheers,