A lot of pattern companies have dress shell patterns (also referred to as slopers) which represent the "standard" woman used to design patterns for that company. For example, see Vogue V1004 or McCalls M2718. The idea is that you'd start with the dress shell pattern and alter it until it fits you perfectly. Traditionally, it's made from gingahm so you can see the grain lines.

For example, you might find that your bust point is an inch above the "standard" bust point, or that you have one shoulder higher than another, or that your waistline is lower in front than in back. You make the sloper fit YOU, then you use it to make alterations to other patterns that you buy, by laying the shell over the purchased pattern and using it as a guide to make the alterations.

Here's an article that explains it in more detail. I think the author is writing for petite women with large busts, which is why she makes the suggestions she does about how to alter the shell.

If you have any talent for drafting your own patterns, your sloper would be your starting point for designing a pattern to fit you.

CMC

Here is a link that might be useful: What is a sloper?