You use immediate values as operands primarily with instructions. In some cases, you can use immediate values with the operands of directives. For instance, you can use immediate values with the .byte directive to load values into the current section.
It is not usually necessary to use the # prefix for directives. Compare the following statements:
ADD R1, #10
.byte 10
In the first statement, the # prefix is necessary to tell the assembler to add the value 10 to R1. In the second statement, however, the # prefix is not used; the assembler expects the operand to be a value and initializes a byte with the value 10.
See Section 5 for more information on the syntax and usage of directives.