Relative Addressing:

For relative addressing, the implicitly referenced register is the program counter (PC). That is, the current instruction address is added to the address field to produce the EA. Thus, the effective address is a displacement relative to the address of the instruction.

Base-Register Addressing:

The reference register contains a memory address, and the address field contains a displacement from that address. The register reference may be explicit or implicit.

In some implimentation, a single segment/base register is employed and is used implicitly. In others, the programmer may choose a register to hold the base address of a segment, and the instruction must reference it explicitly.