A sanción tributaria is the fine the AEAT (Spanish Tax Agency) imposes for breaching a tax obligation, and infringements are classified as minor, serious, or very serious depending on the harm caused and whether there was concealment or fraudulent means. It is not the same as a surcharge: the penalty comes when the tax agency demands or detects the breach; the surcharge applies when you move first and correct things without any formal notice.
The amount can be cut by 30% if you accept the assessment (conformidad) and, on what remains, by another 40% if you pay on time and do not appeal, under article 188 of the Ley General Tributaria. Appealing is legitimate, but it forfeits those reductions.
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