What is custom code? To answer this question, let’s begin with a metaphor. Imagine that code is a vehicle: a blue Honda Civic.
You might say to yourself, I want this blue Honda Civic. So I will put the car together, build the engine, paint it, install all the electronics, get it entirely road-legal and become responsible for driving it and maintaining too. In other words, if the car breaks down, requires enhancements or fixes, then you’d be solely responsible for fixing the issue or would have to hire someone to take that responsibility. That’s the definition of custom code.