clarification needed on ASSIGN statement

Hi ,

when i write " ASSIGN P0000-MASSN to <DATAIN>" the value of p0000-massn is being assigned to <datain>.

But when i did like below iam not getting.

concatenate 'P' '0000' '-' 'MASSN' into x.

assign x to <datain>.

now the value is not getting assigned . why?