FireMonkey offers a lot more alignment options than those offered in the VCL. The Alignment aligns the control within its parent, setting anchors, size and position. The Default is None. To make it easier to think about them, you can group the different types of alignments.
Anchor and fill along edge – these are 4 of the alignments you are most likely familiar with.
Top, Bottom, Left, Right
Like the above, but takes precedence over other alignments
MostBottom, MostTop, MostLeft, MostRight
Fill parent, but preserve aspect ratio – very powerful, especially when applied to one of the new layouts, like the TScaledLayout.
Fit, FitLeft, FitRight
Resize only on one axis (width or height) to fill that axis of the parent and optionally centered
Vertical, VertCenter, Horizontal, HorzCenter
Client – Fills remaining space not occupied by other pinned controls.
Contents – Fills entire client area of the parent overlapping other controls.
Center – Just moves to the center without resizing.
Scale – Maintains relative size and position relative to the parent as it resizes.
Checkout the full DocWiki pages for more details on FMX.Types.TAlignLayout and Vcl.Controls.TAlign.