how load cases are formed in caesar? i dont understand why we first add wind load ( or occasional loads) to operating load cases and then subtract the respective operating load case to get again wind load. this load is now then added to sustained load case which is then used to calculate stresses.

If you have a system with non-linear boundary conditions you can't simply run a load case with "Win1". You "may" get different results then (OPE+Win1) - (OPE). It depends on how the non-linear supports behave.

well my ques was, we run (OPE + WIN1) for restraint summary and for calculating stress we do like this...
lets say L5=(OPE+WIN1)-OPE
L6=SUS+L5.
we calculate stresses for L6.
first of all why can't we make a load case like L6=SUS+Win1
and second why we go for SUS+WIN1? since plant will be in operating conditions when any occasional load will be applied, then why do we neglect the OPE(operating conditions)?

Vineet, for occasional stress limits please refer to B31.3, 302.3.6.a).

"first of all why can't we make a load case like L6=SUS+Win1"

Because the Code says add the stresses from the OCC event to the stresses from the SUS case. The "OCC event" (in your case) is wind, but you want the effects of wind in the operating condition - so how do you get that?

(OPE+WIN1) - (OPE) = the (perhaps non-linear) effects of wind in the OPE condition.

"why we go for SUS+WIN1"
Because you're evaluating a "primary stress failure".

The OPE case is not (typically) a stress compliance case. There is a "sticky" topic at the top of this forum that explains this.