GIPS Input requirement versus recommendation valuation, business, calendar, starting, include

lxwarr30

Standard 1.A.4 states that starting 1/1/2010, firms MUST value portfolios as of calendar month-end or last business day of the month. However, one of the recommendations (Standard 1.B.3) states, calendar month-end valuation or valuation on the last business day is recommended.
So, what is the point of requiring then only to include as part of recommendation? Any thoughts?