What are the situations in which performing the obligatory prayer while seated on a chair is permissible, and is it permissible to pray the supererogatory prayer on a chair while being able to stand?

It is permissible for a Muslim to pray the obligatory prayer while sitting (or on a chair) if they fear harm or severe hardship from standing. If one begins standing and then an excuse arises, it is permissible for them to sit and complete their prayer. As for supererogatory (nafl) prayers, it is permissible to pray them sitting absolutely. For one who prays them sitting while being able to stand, they receive half the reward of one who prays standing. However, if they sit due to an excuse, they receive the full reward.