In CSS box alignment the alignment subject is the thing (or things) being aligned within the alignment container by the property.
For justify-self and align-self, the alignment subject is the margin box of the box the property is set on, using the writing mode of that box. For justify-content and align-content, the writing mode of the box is also used.
The definition of the alignment subject depends on the layout mode being used.
The entire content of the block as a single unit.
The column boxes, with any spacing inserted between column boxes added to the relevant column gaps.
For justify-content, the flex items in each flex line. For align-content, the flex lines. Note, this only has an effect on multi-line flex containers.
The grid tracks in the appropriate axis, with any spacing inserted between tracks added to the relevant gutters. Collapsed gutters are treated as a single opportunity for space insertion.