My name is Hobbyist and you asked a question about the call sign about British Airways BAW99 to BAW9L and here is my interpetation on the answer of the question and I,ll give you an example of it first.
Like some flights that go around the US they have a what you call a slash/ R-Romeo or a slash/ W-Whiskey like NW-Northwest 1298 or NW-Northwest 298/W that means that is how they are set up for radio like navigational system. It is in the navigational system is how they are set up in that form. And here is another example; If this flight is a lifeguard flight they will call it Northwest 1298 lifeguard. That means that they have a body organ in transit to the flight and they have the right of way. I hope that gives you the right interpetation on it. Thanks.
Thanks from Hobbyist.
I think "BAW9L" has nothing to do with the equipment suffix (at least here in the US). Unfortunately, I do not know what it does refer to, but i know that sometimes when airlines already have a flight with a certain flight number airborne and then a flight with the same flight number leaves later/earlier because of delay which means two of the same flight numbers would be flying/airborne, they may attach a letter to the end of the callsign. The equipment suffix is only spoken to a controller when identifying your aircraft type and equipment suffix on initial contact such as if you didn't already have a flight plan filed.
Also, when a lifeguard flight is operating, they attach "lifeguard" to the beginning of the callsign, not the end. ie: "Lifeguard Northwest 1298" or "Lifeguard November 1234-Alpha."