In the following, "+" and "_" are taken as literal plus and underscore characters, respectively. "|" means "or", and "[...]" means that ... is optional.

selectkeys.txt has the format

KEY SOURCE+FILTER+CONCLUSION+

"SOURCE+FILTER+CONCLUSION+" will be called the selector. Generally, all elements of the selector are separated by _, where the + does not count as a separator on its own. You'll see what I mean.

Note that

    * Spaces are significant. Do not insert any spaces in the selector string!
    * Do not use a KEY that has already been bound to something else (this holds for SelectionEditor too). 

The format for KEY is different from uikeys.txt: Use a combination of Shift Control Alt (or nothing) followed by a key A-Z or 0-9, all separated by an underscore _. Example:

Control_W
Alt_Shift_0
T

SOURCE describes the set of units that you want to filter and pick a selection from. It can be one of

    * AllMap: obvious
    * Visible: obvious
    * FromMouse_D, where D is some number: all units that are at most a distance of D away from the mouse cursor
    * FromMouseC_D, same as above, but using a vertical cylinder instead of a sphere -> good for selecting airplanes or ships on deep water
    * PrevSelection: the previous selection; that is, the one active before you hit the selection key 

FILTER is an arbitrarily long list of filters. Every filter can be preceded by Not to negate it. You will still have to use a _ to separate the Not from the filter, as in Not_Commander.

Here are the filters. Note that "units" generally means both buildings and units. Typing both got old real quick.

    * AbsoluteHealth_N: only units that have an absolute health > N
    * Aircraft
    * Builder: only construction units
    * Building: only buildings
    * Category_C: only units of category C -- XXX what are the categories?
    * Commander
    * Idle
    * InHotkeyGroup: only units that are in a group
    * InPrevSel: only units of the same type as a unit in the previous selection
    * NameContain_S: only units whose name contains the string S
    * Radar: only units with either radar, sonar, or a jammer
    * RelativeHealth_P: only units that have health > P percent
    * Transport
    * Waiting: only units that are under a wait command (usually W)
    * WeaponRange_D: only units that have a weapon range > D
    * Weapons: only units that have weapons 

CONCLUSION is

[_ClearSelection]_HOWMANY

If you specify ClearSelection, your new selection will replace the old one; otherwise, it will just add to it. HOWMANY must be exactly one of

    * SelectAll: all units
    * SelectOne: one unit, will also center the camera on that unit
    * SelectNum_N: at most N units -- XXX I think this includes the previously selected units (without ClearSelection), check!
    * SelectPart_P: P percent of the units 

Recall that between every two tokens, there must be an underscore _, even if there is also a +. Another way to put it is that before every word in your selector except the SOURCE, there must be an underscore.

Some examples, first the standard bindings. Again, "unit" also includes buildings.

Control_A AllMap++_ClearSelection_SelectAll+

Selects everything on the entire map.

Control_B AllMap+_Builder_Idle+_ClearSelection_SelectOne+

Selects any (one) idle builder on entire map.

Control_C AllMap+_Commander+_ClearSelection_SelectOne+

Selects your commander.

Control_R AllMap+_Radar+_ClearSelection_SelectAll+

Selects all units with radar/sonar/jammer.

Control_W AllMap+_Not_Aircraft_Weapons+_ClearSelection_SelectAll+

Selects all non-aircraft armed units

Control_Z AllMap+_InPrevSel+_ClearSelection_SelectAll+

Selects all units of a type that was in your previous selection.

Note that up to now, all keys said ClearSelection, hence they replaced your old selection.

Control_X AllMap+_InPrevSel_Not_InHotkeyGroup+_SelectAll+

Selects all units of a type that was in your previous selection, unless they are already in a hotkey group. (Use case: if you have a set of tanks on group 1, and have since produced more of the same type(s), you could hit 1 Ctrl+V Ctrl+1 to include them in the group.)

Control_V AllMap+_Not_Builder_Not_Commander_InPrevSel_Not_InHotkeyGroup+_SelectAll+

Selects all units of a type that was in your previous selection, except builders and commander, unless they are already in a hotkey group.

And finally, an example from the forums:

Control_W PrevSelection+_Not_Building_Not_RelativeHealth_30+_ClearSelection_SelectAll+

From your previous selection, leaves everything that is below 30% health, and not a building. (Use this to quickly retreat damaged units.)