Storage keys of the files written by a process execution.

An execution writes a numbered family of files: the location posts/%{year}/sitemap.xml run with year: 2026 writes posts/2026/sitemap1.xml, posts/2026/sitemap2.xml, ... (with .gz when the index is gzipped). A process without location writes sitemap_<name>1.xml, ... next to the index. When the file name ends with a digit, a - separates it from the counter (posts-12-1.xml). Keys are relative to the index directory and their path segments may only contain A-Z a-z 0-9 . _ ~ -, so they are valid URL paths as they are.