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  

On the Day of Nahr at Mina, the Prophet ﷺ said, "Tomorrow we shall stay at Khaif Bani Kinana where the pagans had taken the oath of Kufr (heathenism)." He meant (by that place) Al-Muhassab where the Quraish tribe and Bani Kinana concluded a contract against Bani Hashim and Bani ʿAbdul-Muttalib or Bani Al-Muttalib that they would not intermarry with them or deal with them in business until they handed over the Prophet ﷺ to them.  

قَالَ النَّبِيُّ ﷺ مِنَ الْغَدِ يَوْمَ النَّحْرِ وَهُوَ بِمِنًى نَحْنُ نَازِلُونَ غَدًا بِخَيْفِ بَنِي كِنَانَةَ حَيْثُ تَقَاسَمُوا عَلَى الْكُفْرِ يَعْنِي ذَلِكَ الْمُحَصَّبَ وَذَلِكَ أَنَّ قُرَيْشًا وَكِنَانَةَ تَحَالَفَتْ عَلَى بَنِي هَاشِمٍ وَبَنِي عَبْدِ الْمُطَّلِبِ أَوْ بَنِي الْمُطَّلِبِ أَنْ لاَ يُنَاكِحُوهُمْ وَلاَ يُبَايِعُوهُمْ حَتَّى يُسْلِمُوا إِلَيْهِمُ النَّبِيَّ ﷺ وَقَالَ سَلاَمَةُ عَنْ عُقَيْلٍ وَيَحْيَى بْنُ الضَّحَّاكِ عَنِ الأَوْزَاعِيِّ أَخْبَرَنِي ابْنُ شِهَابٍ وَقَالاَ بَنِي هَاشِمٍ وَبَنِي الْمُطَّلِبِ قَالَ أَبُو عَبْدِ اللَّهِ بَنِي الْمُطَّلِبِ أَشْبَهُ