The twelfth house is an apoklima, cadent, and its Hellenistic name is Kakos Daimon, the Bad Spirit. It is the joy of Saturn. Counting by whole signs it stands at no aspect to the rising sign, so like the second, sixth and eighth it is in aversion to the Horoskopos, and like the sixth it is cadent as well, carrying both disadvantages at once.
Its most consequential feature is the boundary it shares with the first house, because that boundary moves between systems and the two houses on either side of it are read very differently. In the quadrant systems and in Equal, the twelfth ends at the ascendant degree, so a planet a few degrees before the ascendant is in the twelfth. In whole-sign houses the twelfth is the whole sign before the rising sign, so that same planet, if it is in the rising sign, is in the first house instead.
This is the single most common discrepancy a reader will meet when comparing two charts of the same nativity, and it flips a placement between the most personal house in the figure and the one the tradition reads as hidden, undoing and self-undermining. Neither reading is wrong; they belong to different schemes.
At high latitude there is a further caution. Placidus and Koch are undefined above the polar circles, and near them the twelfth can become extremely large or extremely small.