An immediate operand, also called a constant operand in some contexts, is a bit pattern that is embedded into an instruction in machine code. As such, immediate operands do not use data memory at all, as all instructions should be in code memory.
Furthermore, the value of an immediate operand cannot be changed in most situations, as it is very poor practice to modify instructions (in machine code) when a program executes. Self modifying programs exist for very specialized purposes, such as obfuscation.
Immediate operands are only used as a source operand. This is because an immediate operand cannot be changed (since it is a part of an instruction).
The access speed of immediate operands is fast, as the value of the operand is a part of the binary bit pattern of an instruction.