'Not Mount' on PCB Design
The board is laid out with footprints for the components indicated, but they are not populated by default. Sometimes the acronym DNP (Do Not Populate) is used. 

Some variant of the board can be made with some or all of the indicated parts populated and possibly some other parts not populated. It allows various options of a board to be made with a single PCB pattern. 

Sometimes this makes a great deal of sense- since the PCB is a custom part, you can minimize the number of custom parts by allowing for options that can be purchased by the customer for additional features, or for different models. Sometimes it doesn't, for obvious reasons of increasing complexity, more PCB area and reduced yield, more difficult PCB assembly because parts are crammed in and so on.