The answer was in the document you linked to earlier.
You mean this?
Grade 1 is for a property unlikely to attract an intruder and that any attempted intrusion is likely to be a spur-of-the-moment occurrance with no prior planning.
Properties falling into this Grade would be low grade domestic properties without an insurance requirement.
That's not the whole definition in the PD6662 standard, surely?
PD6662 doesn`t define the grading standard - no document does. As I said it`s not something your going to "get" unless you are trade.