k(rel) - whats this ? FC-catalyst activity related

[Organikum]

I found this in a german script and would urgently like to know what "k(rel)" stands for. It seems to describe relative activity of the catalyst, but what does it say exactly?
It is on Friedels-Craft alkylations with the example cumene production:

Kat/k(rel)
--------------
SbCl4/1300
FeCl3/570
AlCl3/1
SnCl4/0,03


thanks
ORG

[Rhodium]

k = reaction rate constant
k_rel = relative reaction rate

Further reading:

http://www.shodor.org/unchem/advanced/kin/