Files placed in \edawn\maps directory is used to modify map entities such as 
weapons, ammos, powerups, movers etc.

Filenames matches existing map names while extension tells what action to perform:

*.set - replace entire compiled-in map entity set by specified file
*.add - just add new entities to the existing entity set

For example:

q3dm17.add - extension means that specified file will add something new to q3dm17
q3dm13.set - will replace whole entity set so usual items may be changed or even removed

(Ruleset parameters MapEntitySet and MapEntityAdd can be used to override this)

How to create *.add/*.set files?

type \g_dumpents 1   and then  \map_restart

File with name %mapname%.set (q3dm1.set for example) will be created in \edawn\ directory.
Now you can edit it and move to \edawn\maps\ folder. After that it will replace stock q3dm1
entities :)

How to create/add custom spawn spots?

type \devmap q3dm17 or any map you want
move to some potision where you want to place something
type \cg_debugPosition 1

now you can see your world position so if you want to create custom spawn point -
then just create q3dm17.add with following contents:

{
	"classname" "info_player_deathmatch"
	"angle" "-90"
	"spawnflags" "4"
	"origin" "120 1049 600"
}

Angle (second in the row) is displayed on your screen same as coordinates of origin :)