i assume you mean snow foam?
this is a pre-wash stage, theres two different types, its best to leave it to dwell for a few minutes (when most of it has run off) spray on when the car is dry, this will allow to 'stick' to any lose dirt and pull it away from the paintwork
then just hose off, pressure washer isnt necessary at this stage (although PW is needed with the SF lance)
first type is a ph neutral solution, high foaming, very friendly
second type is a tfr solution, not as high foaming, will strip any wax layers and well as any other crud on the car
once hosed off, wash as normal
recommendations...
ph neutral - valet pro ph neutral snow foam - £25 for 5L bottle
tfr - valet pro snow foam combo - £15 for 5L bottle
waxamomo.co.uk
hope this helps?