In respects to programming and the client-server model of software, client code refers to source code for the client side of a software package. Client code contains instructions for connecting to the server, sending commands, receiving commands and acting on received commands. Not only does the client
contain code to talk to the servers, it also contains code to interact with the end user. As such, the client can become quite complex, as any good client software will validate anything that the user does to make sure it's a valid item.
The complexity of client code is determined by the functionality of the client itself. If a client just has an OK button and queries the server for the time, there's not going to be a lot of code. On the other hand, if the client takes user input and does more than just a query to the server, the number of
source code lines and complexity level rises. With added complexity comes added bugs, and that adds up to unhappy users. The last thing any developer wants is an unhappy user.