But when I see the structure indicator it has total 40 characters:
Why is the system allowing me only 30 characters label?
Question: Why it is showing 40 characters structure indicator in IMG. If it is only 30 character.
We don't have alternate labeling turned on and its not recommended by SAP.
If you don't have alternate labeling turned on then the maximum label size is 30 characters. Is it turned on? If so then where are you getting the error, in IL01?
Functional location labels are an interesting addition to SAP. The table that stores the functional location (IFLOT) uses a key (TPLNR) that is defined as 30 characters long. If you have alternate labeling turned OFF (or using an older version of SAP) then SAP uses this key as the actual label, hence its 30 character limit.
However, if you have alternate labeling turned on then you are allowed to use multiple labels to represent the same functional location (as well as renaming labels). For this added functionality SAP has another table called IFLOS which uses a different field as the new label (STRNO). This new field is defined as 40 characters. With alternate labeling turned on SAP gives your new functional location an internal ID and assigns it to TPLNR. The name you gave becomes the label and is stored as STRNO in IFLOS.
So, to support the longer label size the structure indicator edit mask is defined as 40 characters also. I agree that SAP should've put a warning in there if you save a structure mask longer than 30 characters and have alternate labeling turned off but they don't and you just have to know I guess.