Source SDK Base 2007 is a game/tool, shared between all owners of Valve Source games and locked to a particular engine branch, on which modders can base their projects.
Both singleplayer and multiplayer mods can use it.
The stock gamecode is a duplicate of Episode One's or Episode Two's, depending on which version (see below) you are using.
But the only content available is that of Half-Life 2 and Half-Life 2: Lost Coast: while Zombines and Hunters exist in the code and can be spawned, they appear as errors because their models are not available.
If you want to use episodic things, you will need to mount the appropriate game.
SDK Base might provide mods with Lost Coast's content, but dedicated servers do not. Multiplayer mods should either not use anything from Lost Coast, or include what they do use in their dedicated server download.
Source SDK Base - unique software, shared between all owners of Valve Source games that provides mods with Lost Coast's content and allows you to mount games.