Mappers seems to follow this scale when they bother to map piste:difficulty on skitour, at least there is no outstanding use of ‘freeride’. Below are the numbers for ways, then relation in Opensnowmap rendering database.

pistes_imposm=# select count(*), difficulty from osm_pistes_ways where piste_type='skitour' group by difficulty;
 count |  difficulty  
-------+--------------
  2801 | 
   224 | advanced
   978 | easy
    92 | expert
    56 | extreme
   177 | freeride
   667 | intermediate
     4 | L
    41 | novice
     3 | WS
     1 | WS-
(11 lignes)

pistes_imposm=# select count(*), difficulty from osm_pistes_routes where piste_type='skitour' group by difficulty;
 count |  difficulty  
-------+--------------
     2 | novice
    53 | easy
    43 | intermediate
    16 | advanced
   451 | 
     5 | expert