Hi @a238livemix ,
FX engines 1-4 and 5-8 are only different in that you can send to 1-4 using the FX send busses.
So with the default setup, FX engines 1-4 are fed via the FX sends and have some commonly used FX units already loaded.
FX engines 5-8 do not have units loaded and are not routed.
Any of the FX engines can be used inserted on any input or mix channel though, assigned either by using the ‘Back Panel’ in the FX screen, or from the insert points in processing.
It’s also possible to use any/all of the engines in a ‘Mix -> Return’ setup (https://www.allen-heath.com/media/SQ_ReferenceGuide_V1_5_0.pdf#page=53&zoom=auto,-361,567) which allows you to feed them from any mix output for sends on faders, or an input direct out. As soon as the returns are in use with this mode, they will be visible on channel strips wherever you have placed them.
(when not in ‘Mix -> Return’, you can place the FX returns on channel strips, but they will not show up on the surface as they are not doing anything)
Hope this helps!
Keith.