An expression specifies an address or numeric value. White-space may precede and/or follow an expression.
The result of an expression must be an absolute number, or else an offset into a particular section. If an expression is not absolute, and there is not enough information when the assembler sees the expression to know its section, a second pass over the source program might be necessary to interpret the expression—but the second pass is currently not implemented. The assembler aborts with an error message in this situation.
An empty expression has no value: it is just white-space or null. Wherever an absolute expression is required, you may omit the expression, and the assembler assumes a value of (absolute) 0. This is compatible with other assemblers.