sm_reserve_type 0
Public slots are used in preference to reserved slots. Reserved slots are freed before public slots. No players are ever kicked and once reserved slots are filled by a reserve slot player (and the rest of the server is full) they will remain occupied until a player leaves. The use of this is that there can always be at least one admin (assuming you only give reserved slots to admins) on the server at any time. If players inform you that there is a hacker on the server, at least one admin should be able to get it and do something about it. If a player without reserve slot access joins when there are only reserved spaces remaining they will be kicked from the server.
sm_reserve_type 1
If someone with reserve access joins into a reserved slot, the player with the highest latency and without reserve access (spectator players are selected first) is kicked to make room. Thus, the reserved slots always remain free. The only situation where the reserved slot(s) can become properly occupied is if the server is full with reserve slot access clients. This is for servers that want some people to have playing preference over other. With this method admins could one by one join a full server until they all get in.
sm_reserve_type 2 - Only available in SourceMod 1.1 or higher.
The same as sm_reserve_type 1 except once a certain number of admins have been reached the reserve slot stops kicking people and anyone can join to fill the server. You can use this to simulate having a large number of reserved slots with sm_reserve_type 0 but with only need to have 1 slot unavailable when there are less admins connected.