Hi,
Put very simply Absolute is when your User Registers are assigned fixed locations in memory; typically used when you are doing simple one module programs.
With Relocatable code you let the system determine where to place the User Registers, it is typically used when writing code in several different modules that can be added or removed as needed.
Each module just lets the system know how many and what the User Registers are called; the complier then allocates the memory locations for you.