Some airlines (such as AA) have different types of 'A' and 'C' inventory, cash and upgrade, but what has been a topic here in recent years is a fairly new issue of point of commencement/origination and maybe type of fare restrictions. It's the same 'A' or 'D' or 'L' inventory but the amount available can be restricted based on where the first flight in the record originates or (possibly) the type of fare. It's more similar to married segment availability than to upgrade vs revenue inventory. It's true that inside the GDS there isn't just the single-letter inventory that we can see using EF or whatever; and this might be the mechanism that us used for implementation, but that's merely a point of interest.