Slice position in RAS is stored in the last (4th column) of the SliceToRAS matrix stored in the slice node.
Slice position increment equals acquisition slice spacing when you switch to previous/next slice using arrow keys or page up/down keys on the keyboard. However, when you move the slice positioning slider using your mouse then you can set the slice position at higher precision. Slicer interpolates between slices by default, but you can disable this (see for example this topic for more info). You can still set the slice position to be between original acquisition slices, but the voxels values will not be interpolated for display.