The epígrafe IAE is the activity code you declare when registering with the tax agency on form 036, taken from the IAE (economic activities tax) classification. Choosing the right one matters, because it drives things like the income tax withholding on your invoices or the VAT treatment, even though most businesses pay no IAE fee at all.
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.
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