It is space that is allocated based on some specific placement commands.
Look at these commands for the IAR XLINK Linker:
These commands will allocate the segment MYSEGMENT on address 4000 and upwards. 0x8000 (SPACE) additional bytes will also be allocated at the end of MYSEGMENT. If MYSEGMENT had a size of 0x45678 bytes it will show up as 0x4D678 bytes (0x45678 + 0x8000) in the segment map (it will occupy 0x4000 - 0x51677), the module map will show N/A (command line) 32 768 (0x8000) if SPACE was the only extra space.
This is listed in this way as the extra space allocated in the placement commands isn't a part of the program, they do not reside in a module, they are just there because they were defined on the command line. This is primarily used to set the size of the stack and the heap in an easily configurable way.
The name is due to the "Linker point of view". The Linker doesn't know of any Compiler and/or Assembler tools. The Linker just link "relocatable CODE and DATA modules" into output, placed in the available memory. As the DATA (that we discuss) is not relocatable, it enters the Linker in another fashion. And from the "Linker point of view" the "fixed placed DATA" is additional input from the command line.
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