When do pilots hold behind ILS Critical area?
In the US (and you're looking at an FAA manual, so that implies US rules), this is covered in JO 7110.65, the FAA order covering air traffic control procedures. Flip to page 135 of that PDF (note that the linked version is not yet the currently effective version) for Section 3-7-5:


  3−7−5.  PRECISION APPROACH CRITICALAREA a.ILS  critical  area 
  dimensions  are  described  in FAA  Order  6750.16,  Siting  Criteria 
  for  Instrument Landing Systems. Aircraft and vehicle access to the
  ILS  critical  area  must  be  controlled  to  ensure  the integrity
  of ILS course signals whenever the official weather observation is a
  ceiling of less than 800 feet or  visibility  less  than  2  miles. 
  Do  not  authorize vehicles/aircraft to operate in or over the
  critical area, except  as  specified  in  subpara  a1,  whenever  an
  arriving aircraft is inside the ILS outer marker (OM)or the fix used
  in lieu of the OM unless the arriving aircraft has reported the runway
  in sight or is circling to land on another runway.
  
  PHRASEOLOGY−HOLD SHORT OF (runway) ILS CRITICAL AREA


It goes on from there to describe further details, including instructions on when to keep the critical area clear and instructions to warn aircraft conducting autoland approaches if the critical area will not be protected. 

In short, pilots hold short of the critical area if they're instructed to do so, and controllers tell pilots to do that by simply saying "HOLD SHORT OF (runway) ILS CRITICAL AREA."