Slot purposes on Apple II computers

Apple II computers come with 7 expansion slots built into their motherboards.
While there is some flexibility in what each of these slots can do, they are
generally intended for a relatively narrow, specific function.

When the Apple II boots, it scans the slots for bootable devices from the
highest-numbered slots downward. In other words, it first tries to boot from
slot 7. If this fails, it tries to boot from slot 6, and failing that, it
tries to boot from slot 5. This is why the primary boot device is typically
on slot 6, and a secondary drive is on slot 5.

The most typical slot purposes are listed below:

Slot 1: Printer interface card
Slot 2: Modem
Slot 3: On the original Apple II and Apple II Plus, this slot is usually used
by the 80-column expansion card to enable 80-column display mode. On the
Apple IIe, this slot is usually left empty, as the IIe has an auxiliary
expansion slot which is designed to accept an expansion card which enables
80-column mode, and the auxiliary slot overrides most cards in slot 3. On the
Apple IIgs, 80-column display support is built into the firmware and no
expansion card is required for it, but slot 3 is still reserved for this
purpose (typing "PR#3" at the Applesoft BASIC prompt will put the IIgs into
80-column mode) and thus it should be avoided for most types of expansion
cards except those which will not interact with the computer's firmware.
Slot 4: Mouse or memory expansion card
Slot 5: 3.5" disk drive interface card ("Smart Port" on the IIgs)
Slot 6: 5.25" disk drive interface card ("Disk Port" on the IIgs)
Slot 7: AppleTalk card or other expansion card ("Your Card" on the IIgs)