Individuals are exempt from the IAE fee, and companies are too until their turnover reaches 1,000,000 euros; declaring the code is mandatory regardless. You can hold several codes at once, and they can be updated with the same form 036 if your activity changes.
In practice the IAE tables separate business, professional, and artistic activities. The difference matters: if your code is a professional one and you invoice companies or other autónomos, you normally apply IRPF withholding on your invoices; if it is a business code, as a rule you do not. It also affects whether you can use special VAT schemes or the module-based system.
Typical mistake: picking a code that sounds close without reading its description, or keeping the old one after your activity changes. A designer registered under a retail code, for example, may end up invoicing without the withholding the client expected. If the tax agency finds that the code does not match what you actually do, it can adjust withholding or VAT.
Do not confuse it with the CNAE, the classification used by Social Security and for statistics. The epígrafe is linked to withholding, VAT, and your quarterly returns. In kontora returns are prepared box by box with deadline reminders, based on the activity you have registered.
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