OPC 43 means that the cement is expected to reach a compressive strength of 43 MPa in a cube cast by 28 days. Similarly, OPC 53 reaches 53 MPa by 28 days.
There is a contrasting difference in the setting times of these cements. OPC 53 sets quicker than OPC 43 and has a quite low initial setting time. It is used in structures where rapid strength gain is required like large load bearing structures like bridges, huge buildings, etc. OPC 43 on the other hand doesn’t set so quickly and has a normal initial and final setting time. It is hence used where rapid hardening is not necessary like flooring, plastering, etc.