Although it is a full circle from the OP's question, here's a familiar sample case. EASA rules.
Your destination WS is ok, however company preferred
alternate is not looking great. TAF LIMC (MXP) V2kt 2000 BR NSC 3/1 Q1003 TEMPO 14-18 0600 FG BECMG 20-24 220/5 6000
A well equipped major hub with dual runways, CATIIIB and given the forecast it still is a respectable choice. BTW destination is CAVOK, and a non-preferred alternate is 8000 m VIS half way between DEST and LIMC.
Alternate planning minima is one grade more restrictive than what's available, i.e. for Milano CAT I ILS R550 / VIS 800.
As much as we agreed the paragraphs we're looking at do not allow conversion of TAF 600 VIS to 900 CMV, because the target minimum is R550 (less than 800) ...
... when you ran the above scenario through the flight planning system, the answer is ACCEPTABLE.
Where's the catch?
