It (i.e. Al-Abtah) was a place where the Prophet ﷺ used to camp so that it might be easier for him to depart.  

إِنَّمَا كَانَ مَنْزِلٌ يَنْزِلُهُ النَّبِيُّ ﷺ لِيَكُونَ أَسْمَحَ لِخُرُوجِهِ يَعْنِي بِالأَبْطَحِ  

Staying at Al-Mahassab is not one of the ceremonies (of Hajj), but Al-Mahassab is a place where Messenger of Allah ﷺ camped (during his Hajjat-al-Wida).  

عَنِ ابْنِ عَبَّاسٍ ؓ لَيْسَ التَّحْصِيبُ بِشَىْءٍ إِنَّمَا هُوَ مَنْزِلٌ نَزَلَهُ رَسُولُ اللَّهِ ﷺ