I am after putting a 1.6 duratec in my car in rwd form but im not really sure what the difference is between the engine codes can anybody help?

I know the 1.6 duratec is just a rebadged slightly updated 1.6/1.7 Zetec SE or known as the sigma engine designed by Yamaha not to be confused with either the Zetec E or the Duratec 2.0 or bigger.

What I don’t understand is in the latest variant of the 1.6 Duratec/Sigma engine is the 4 letter code attached to them and what the differences are?

HXJA
HXJB
HXDA
HWDA
SHDA

I will be fitting one with the variable cams (TIVCT) I will be using individual throttle bodies and an OMEX ECU so not fussed if the only difference is the way ford tune them but if its better heads or different valves ect I want to know this before buying one so I can get the best one for power they all look the same from the pictures on adverts on ebay so cannot see an external difference apart from the ones with TIVCT

Cheers guys for your responses