In the example below, all the bold entries must be unique for each variation and the [fltsim.X] must follow on consecutively 0, 1, 2, 3, 4 etc.
I find that the title needs a unique name too, hence the Paint1, Paint2 etc.
description=From the most successful family of jetliners ever built comes the 737-400. This can be your entry into virtual airline flying. Whether you're making short-haul or longer domestic flights, the 737-400 is a versatile and efficient airplane. See the Aircraft Information section of Help for tips on flying this aircraft.
title=Boeing 737-400 Paint1
ui_variation=British Airways 'Benyhone Tartan'
description=G-BVNO, a Boeing 737-4S3, delivered 30th June 1989. Currently painted in the Benyhone Tartan colour scheme.