The documentation you want is for ld65 (the linker), section 5:
https://cc65.github.io/doc/ld65.html#s5
1.
file="" means don't create a file, i.e. do not make a separate file with output from this MEMORY block. This is important for defining areas of memory that don't need to be written to disk; ZP and RAM areas, as well as the stack (if defined separately) are good examples, e.g. things that are not going to be part of the resulting ROM.
file=%O means create a file based on the output filename of what's passed via
-o flag to the linker. You can likewise say
file="myfile.bin" and so on. In "simple cases" this is usually the resulting .NES file, but every project differs.
2. The line in question, which is in the MEMORY section, ends up going into the resulting ROM file because of the
file=%O directive.
How this ends up being the PPU pattern table (PPU RAM $0000-1FFF) is purely due to the .NES file format itself. The file format is, summarised:
First: a 16-byte header (which includes the number of 16KB PRG banks, and the number of 8KB CHR-ROM banks)
Second: all of the PRG banks
Third: all of the CHR banks
This is why for MEMORY, you'll see things like
HEADER with
file=%O,
ROMn with
file=%O, and
CHR with
file=%O.
%O essentially means "the resulting .NES file".
Your
SEGMENTS section will have references to things in the
MEMORY section using directives like
load=THING_IN_MEMORY_BLOCK, e.g.
CODE: load=ROMn, type=ro and
VECTORS: load=ROMn, type=ro, start=$FFFA.
For games that are CHR-RAM, there are no CHR banks (the header will have 0 for the CHR bank count, and there will be no CHR data at the end of the .NES file). Those games use native 6502 code to copy the CHR data (from somewhere in PRG ROM space) into PPU RAM $0000-1FFF (because it's actually RAM on the cartridge, not ROM on the cartridge).
I still maintain this aspect of the ca65 suite is one of the biggest stumbling blocks about the assembler. The old classic concatenation method, e.g.
copy /b header.bin+prg.bin+chr.bin mygame.nes makes a lot more sense to people when they see it.