Generally, a front post stitch is worked around a stitch that was made in a previous round or row. The front post is anchored around the stem and naturally pulled upward, pulling from the stem towards the top end of the stitch it is worked around.
When working around a stitch that is made in the same round, the direction of the stitch pull is no longer upwards, but usually directionally opposite to the stitch it is worked around. The affix ‘-anc’ is added to indicate that the stitch is worked around the anchored (base) end of the stitch and that the point of anchoring in relation to the next st is used as the reference for the stitch placement description, not when the stitch we are supposed to work around was made.