وَيَلْزَمُهُ سُجُودُ السَّهْوِ إِذَا زَادَ فِي صَلَاتِهِ فِعْلًا مِنْ جِنْسِهَا لَيْسَ مِنْهَا، أَوْ تَرَكَ فِعْلًا مَسْنُونًا،
Translation: Sujud al-sahw becomes due when someone inadvertently adds an act of the same type as the prayer but not belonging at that point, or omits a prescribed Sunnah-established act of the class that requires compensation.
An addition of the same type means an extra prayer act, such as an unwarranted bow or prostration. A separate kind of action that invalidates prayer must be considered under its own ruling.
The phrase a Sunnah-established act does not mean that every ordinary Sunnah omitted triggers sujud al-sahw; it concerns specifically binding practices established by Sunnah.