The meaning of the "Score" in ScanProsite
The score provides an indication of well a specific profile (which is a weight matrix) matches the profile for the domain. Section II.B. Profiles development explains this in further detail. 

A paper that outlines the development of PROSITE notes that they primarily use a cut-off of 8.5 and explains the score thus;  


  the raw scores in PROSITE profiles are converted into so-called ‘log10
  per residue E-values’, allowing the computation of the number of
  expected matches with equal or higher score in a database of a given
  size. For instance a match with a normalised score of 9.0 or higher is
  expected to occur about once in a database of one billion residues.