Although it's the source code for the Atari 800 version of PCS, it seems to have been built on an Apple II. From MAKE1 [1] for example:

    CALL -151
    3F2:69 FF 5A
    BLOAD GPAK.OBJ,A$1300
    BLOAD CDRAW.O,A$2480
    BLOAD PCSA800.PIC,A$2B00
    BLOAD DLIST.O,A$4910
    BLOAD SWAP.O,D2,A$4A00
    BLOAD TST.O,D1,A$4B00
    BLOAD GOATARI.O,D2,A$4B45
    BLOAD BOOT.O,D2,A$8E00
    BLOAD BOOT2.O,A$8E80
    BLOAD ZAP1.O,A$8F00
    BLOAD DOWNLOAD.O,D1,A$300

The first line invokes the Apple II "System Monitor", the second writes 3 bytes to memory at address $3F2 (the effect of which is to tell the computer to enter the Monitor whenever someone resets the computer from the keyboard), and the rest load binary files into different memory areas. I'm guessing it's all to build an image to be written to disk. (See [2] for more on the System Monitor.)

[1] https://github.com/billbudge/PCS_Atari800/blob/master/Disk1/...

[2] http://apple2.info/wiki/index.php?title=System_Monitor#Gener...

EDIT: added additional details