A permanent link to an item (a blog post, an attachment, ...). The default structure is "?p=123", but WordPress can handle "pretty permalinks" and will rewrite them internally to the former version. If you have a question about special rewrite structures you should use the [url-rewriting] tag.
I'm stuck for some days with this problem and I don't know what else to do. With %somethingHere% on slug rewrite, pages permalinks are broken (CPT AND CT OK). Without, I don't have my desired Post ...