For original man-page of fail2ban configuration use man jail.conf or e. g. here - https://manned.org/jail.conf.5 How the default basic interpolation in python config files works you can read in the python documentation (e. g. see https://docs.python.org/3/library/configparser.html#supported-ini-file-structure)

Our implementation is more complex (allows includes, recursive interlaced substitution, etc.).

[Q] I'm trying to reference some variables in my configuration, but cftoken = %(cftoken)s doesn't work

[A] This case cftoken = %(cftoken)s is not allowed, because it will produce endless cycle (always substitute itself, again and again).

As well as following is not allowed also (for the same reason):

A = %(B)s 
B = %(C)s
C = %(A)s

Since we support an extensions called "known" with syntax %(known/option)s, after some fix (introduced somewhere 0.9.3 or 0.9.4, but some features possible first in 0.10), it allows to use the same last known option...

So try your luck with cftoken = %(known/cftoken)s

If you will set or deliver some init parameter from jail to filter or action, you can currently use another interpolation extension of us with another syntax <option> (Note: this is a run-time substitution):

# action my-action.conf
[Definition]
something_using_cftoken = command --token="<cftoken>"
[Init]
cftoken = default value

# jail.local
[my-jail]
banaction = my-action[cftoken="customized value"]