commandToClient(%client, %func, %arg1, %arg2, ...); is a server-sided function which essentially calls a function on the specified client's client equal to clientCmd(insert %func here)(%arg1, %arg2, ...);.
There is no client object passed because it won't exist on the client and there's no reason for it to. Any information about a client on the server you want to pass to a client, you have to do through individual arguments or a type of string containing all the information (e.g. having numerical values in a sequence of words wherein all words of the same index always indicate the same value)
commandToServer(%func, %arg1, %arg2, ...); is a client-sided function which essentially calls a function on the server equal to serverCmd(insert %func here)(client object that called commandToServer, %arg1, %arg2, ...);.
When you do a /cmd in chat, your client is converting that into a commandToServer call.
Keep in mind, the %func arguments have to be tags.