DEC BASIC: can FILL$ store spaces

Hello All
Consider the following MAP.

MAP (ABC) &
STRING A$ = 3%, &
STRING FILL$ = 3%

When I use this MAP file to write to an sequential file, I see SPECIAL characters for FILL. For example:
XYZ***
YUX***

I have used * to represent the special character.

My question is: How do I make FILL print SPACES?

I understand I could just as easlily use a variable and assign spaces to it like so:

MAP (ABC) &
STRING A$ = 3%, &
STRING B$ = 3%

and in BASIC: B$ = SPACE$(3%)

P.S: Why do we need FILLs? From the manual: "FILL items reserve space in map and common blocks and in record buffers accessed by MOVE or REMAP statements." Why? For temporary storage?

Thanks!
.

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