Edit: Mick already explained.
The first one. Previously the standard 1,000 feet vertically was the 'norm' until above FL 290 (29,000 feet) where it became 2,000 feet vertically...hence, FL 310, FL 330, FL 350 etc. FL 310 was "westbound", FL 350 "eastbound", and they alternated.
RVSM standards (improvements in the pitot/static systems and altimeter accuracy) allows for the 1,000 feet minimum now up to (IIRC, FL 430...I never operated a passenger jet that had that high a Service Ceiling). Now, as is the standard at lower levels, even numbered altitudes are "westbound", odd are "eastbound" (westbound/eastbound refers to the actual course of travel, relative to magnetic north...001° to 180° is "east", and 181° to 359° is "west").
RVSM does allow a significant increase in the ability of ATC to handle more airplanes at the altitudes where (coincidentally) contrails are most likely to form, when (of course) conditions are suitable. (**)
.....also (if '@vindog' is reading .....This (**) issue of "suitability" refers to the conditions aloft, in terms of water saturation, temperature.
I had noticed 'vindog' mentioned that contrails (incorrectly calling them "chem"trails, BTW) sometimes do not appear in his local area. This is why they are not seen every day!