Anyone knows if IRAT reselection or redirection needs relation or not?
Yes, need relation.
There are parameter to decide if you want Handover or Redirection for that relation.
3G to 4G redirection - you need LTE relation specifying the eARFCN, BW, etc.
No specific PCI/cell ID is required.
Is it from 4G to other RAT that you are asking?
Even 4G to 3G RwR no relation needed.
Yes, for both direction frequency relation must exist, cell relation is not mandatory.
Reselection can be done with frequency-only relation.
Redirection for coverage-based reason, usually is also based on frequency relation. If you are doing for CSFB, SIB tunneling is a good way to gain some time. BTW, nothing prohibits SIB tunneling for other reasons.