@User standard cells of a given library have all the same height (in some rare cases there are also some double height cells). This allows to place the standard cells in rows. Top horizontal row is a VDD (or GND) supply rail, bottom row is a GND (or VDD) rail. In between there is room for a certain number of horizontal metal 1 signal rows. A library of cells allowing 11 metal1 tracks is easier to route at the expense of some layout space. The high density (hd) library cells allow less metal 1 signals because they are smaller, lower height; less area but harder to route. In some cases extra space must be introduced to allow routing.