What is an allowable fee?

Can you explain what an allowable fee is in the context of cryptocurrency and finance? I'm particularly interested in understanding how it applies to transactions, such as exchanges or trading platforms. Are there any standards or regulations that govern what constitutes an allowable fee, and how do these vary by region or country? Additionally, what are some examples of fees that may not be considered allowable, and how can users avoid them to ensure their transactions are cost-effective and compliant?