وَيُصَلِّي النَّافِلَةَ قَاعِدًا مَعَ الْقُدْرَةِ عَلَى الْقِيَامِ.
Translation: One may perform voluntary prayer while seated even if one is capable of standing.
This is a concession for voluntary prayer and differs from the standing requirement in a fard prayer when standing is possible.
A hadith states: صَلَاةُ الْقَاعِدِ عَلَى النِّصْفِ مِنْ صَلَاةِ الْقَائِمِ
Translation: The prayer of one seated has half the reward of one standing.
This general comparison concerns optional sitting without excuse; a genuine disability has a separate ruling.