TYPE = SLIDE (default) will allow the contact surfaces to transfer pressure (contact closed), but it will not transfer anything under tension (contact open). For lateral forces, the SLIDE option will transfer loads until the sliding phase (lateral force > friction coefficient x pressure)
TYPE=STICK is interpreted in OptiStruct as an enforced stick condition - such contact interfaces will not enter the sliding phase. The contact will still iterate to transfer pressures with an "infinite friction" coefficient and can also iterate to re-open, if tension forces occur at a later iteration (unlike the "no separation" option known from 3rd party solvers).
TYPE=STICK is interpreted in OptiStruct as an enforced stick condition - such contact interfaces will not enter the sliding phase. Also, rotations at the slave node are matched to the rotations of the master patch. The FREEZE condition applies to all respective contact elements, no matter whether open or closed.
This means basically, that the two contact surfaces are "glued" and will transfer all loads / motions between master and slave (also known as Tied Contact). Usecase is to connect two parts entirely.