ICL - Chassi - Only available in DCX switches.
ISL - Switch - In all FC ports.
ICL - Special connector and cable and max 16 x 8 Gbps per cable
ISL - SFP+ and max 16Gbps
More info about ICL here: http://www.brocade.com/downloads/documents/technical_briefs/DCX_ICL_Connectivity_GA-TB-159-00.pdf
The technical differences have already been covered so I’ll just add a few comments from a SAN design perspective.
The key advantages of ICLs are that they don’t use any front side ports and typical ICL configurations have considerably more B/W than would be reasonably possible with ISL designs. ICLs still connect to the switch ASICs as an ISL would and the switches on either side of the ICLs still have a different domain but from a high level SAN design perspective I like to think of them as extensions of the backplane. Just as ISLs, you can put them in a base switch and share the B/W with some or all logical switches in the chassis.
if remember well , when you use ICL then max. number of switches in fabric is 3
the max. amount - depend of DCX model 4 or 8 - of ICL is 16 in 8510-8
in such case with both activated ICL POD License up to eight other Chassis can become Member of the Fabric.
First DCX Generation support up to 4 ICL.