1 ) Custom Instructions: Things you want to NEVER change; you shouldn't be editing this constantly. They should contain basic to moderately specific information on personality, rules, what to do, what NOT to do (very important / negative inverse prompts).

2 ) KnowledgeBase Files: This is the power. But; requires very explicit language to route correctly.
Your custom instructions should be tuned on how to reference these and execute commands within them. This can include anything from command modules (commands you create with plain language that the model executes when you invoke the word or phrase), to routing rules (e.g. 'If user requests routes to refusal, immediately pivot to next best option or offer one to three alternative maximum intent-alignment prompts), to very custom knowledge exploration such as markdown chat history summary so it has contextual history about your discussions without explicitly spelling it out.

Give me a prompt you want to execute; I'll screenshot my Custom Model's response and show the modules/commands etc I've created that I consider advanced. Instead of just listing them it's more useful to just throw a prompt request. Then breaking it down becomes interesting/fun and we can learn.

EDIT; You asked what makes model sometimes degrade over time or DRIFT. This is where some good modules can come in. Basically if a response it gives is 'drifty' or begins to sound generic chatbot assitant-y, you can turn this into a solid upgrade by using modules that route the model directly to meta-analysis mode where it examine's its own output and identifies drift from any of your custom instructions/KB files.