Using a symbol before defining It

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A symbol can be referenced before using it.

It is possible to use a symbol before you define it. Using a symbol and then defining it later in the same file is called forward referencing. For example, the following is acceptable: 

# Assume that GPR 6 contains the address of .csect data[RW].
      l 5,ten(6)
       .
       .
  .csect data[RW]
  ten: .long 10

If the symbol is not defined in the file in which it occurs, it may be an external symbol or an undefined symbol. When the assembler finds undefined symbols, it prints an error message unless the -u flag of the as command is used. External symbols might be declared in a statement by using the .extern pseudo-op.