There are exactly 2 consoles. The client console and the server console.
register_clcmd registers a command may be called from ONLY the client console.
register_svrcmd registers a command may be called from ONLY the server console.
register_concmd registers a command may be called from both the client console AND the server console.
An example of a command registered using register_clcmd is "amx_slapmenu." Typing this into the client console gives the client a menu. Typing it into the server console does not work.
An example of a command registered using register_concmd is "amx_slap." Typing this into the client console slaps a player. Typing it into the server also slaps a player. You can use it in both consoles.