You don't need a Nested Select for this, but just with a Simple Expression. Actually not quite simple, but doable:
when <sales> then
(select tdsls012.dsca from tdsls012, tdsls400
where tdsls400.orno = cprrp100.cdf_<????> and tdsls400.cofc refers to tdsls012)
when <service> then
(select tcmcs065.dsca from tcmcs065, tssoc200
where tssoc200.orno = cprrp100.cdf_<????> and tssoc200.cwoc refers to tcmcs065)
Of course you need to replace the strings between < and >.