Other code letters you will see with some plants, in particular some vegetables such as vine crops and tomatoes, are ones referring to disease resistance. Choose these varieties, and you may have fewer diseases in the garden. In one catalog I saw over 50 listed - not all of course for one crop. Some of the main ones to watch for on tomatoes for instance are TMV (tobacco mosaic virus), TSWV (tomato spotted wilt virus), V (verticillium wilt), and F (fusarium wilt). If there has been late blight in your area in recent years, wiping out tomatoes, look for the few with resistance to this (LB).
The other key point in seed catalog descriptions is days to maturity. This could mean from sowing, or in the case of slow crops the days from setting out plants. Check the catalog to make sure what is meant. This is particularly important in northern areas with short growing seasons (days between frosts), in order to get flowers or in the case of vegetables their fruits. Even with this, if a summer is particularly cool and the crop likes warmth, it may mature slowly.
Descriptions are useful for specifics such as fruit or flower color, particular flavors of vegetables, heights and spreads. Even these may vary greatly among catalogs, so compare several, and they may vary from your own garden. Beware of general and glowing adjectives such as "good", "popular", or "large", as these are relative and may have little meaning in your own garden. Just as the photos are often "enhanced" (don't get disappointed if your flowers and vegetables don't look as luscious), so are many descriptions.
To avoid ending up with too many seeds, roughly map out your garden to scale, then "fit in" the varieties you want grow. A good catalog will give the approximate seeds per packet, and spacing when planting seedlings or sowing seeds. So, for instance, for sweet corn you may see 150 seeds per packet. If the recommendation is to plant 3 seeds per foot, this packet would sow 50 feet of row.