The "priority" option tell witch label will be displayed if two label overlap but if you set the option to render all label even overlaping label, the Z index option let you choose witch one will be on top and will be more likely to be readable.

The "priority" option tells which label will be displayed if two labels overlap but if you set the option to render all labels (even overlapping labels), the Z index option lets you choose which one will be on top and will be more likely to be readable.