it depends:
A scalar value function (SVF ) is called like
Select SVFSchema.SVF( value_to_be_used )
A table value function (TVF ) is called like
Select x.col1, ...
from TVFSchema.TVF( value_to_be_used ) x
-- or
Select x.col1, T.columnA, ...
from mytable T
cross apply TVFSchema.TVF( T.columnY ) x