Why does .bss section load to UFM flash memory in MAX10 QSYS project?

Each of section can be put to different memory regions by user through linker's memory map. ​Refer to https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/nios2/n2sw_nii5v2gen2.pdf, 7.15.2.1 and figure 12

.text—All remaining code

.rodata—The read-only data

.rwdata—Read-write data

.bss—Zero-initialized data