The second paragraph of RenderStack's documentation says that:

/// The final location of non-positioned children is determined by the alignment
/// parameter. The left of each non-positioned child becomes the
/// difference between the child's width and the stack's width scaled by
/// alignment.x. The top of each non-positioned child is computed
/// similarly and scaled by alignment.y. So if the alignment x and y properties
/// are 0.0 (the default) then the non-positioned children remain in the
/// upper-left corner. If the alignment x and y properties are 0.5 then the
/// non-positioned children are centered within the stack.

Sounds like it describes alignment for FractionalOffset. Current implementation uses Alignment.

I am not a native English speaker, and feels hard to express the intention.