Alignment subject

In CSS Box Alignment the alignment subject is the thing (or things) being aligned 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.

Block containers (including table cells): The entire content of the block as a single unit.
Multicol containers: The column boxes, with any spacing inserted between column boxes added to the relevant column gaps.
Flex containers: 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.
Grid 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.