A1 Story codes are a group of symbols on the subject line of stories posted in alt.sex.stories and alt.sex.stories.moderated and several closely-related newsgroups. They communicate to some of the readers some of the contents of the story. Generally, codes are intended to address the download decisions of many potential readers. Some warn of content that some readers want to avoid. Some advertise content that some readers want to download.
Most codes do both.
If you are new to coding, you should probably browse through it once. For a new story, and especially for a new subgenre, you can check the section involved. You should know the codes for sex, age, and number in part 3.0 below. You should be aware of the hot codes in part 3.1. And you should know the codes of your own subgenre.