The default way of viewing the screen shows the designation of your
own sectors, altitude markings for unowned sectors, and nation marks
for other nations' sectors. But there are a lot of different ways of
looking at the map, and of highlighting sectors on it. The [d]isplay
command will list all these for you.
If you are a land race, water sectors are usually marked as a ' ', but
this can be toggled with the [W]ater toggle option: if you
choose this mode, then land sectors will appear as a '.', and water
sectors will be shown in detail.