You guys have to stop copying and pasting documentation from other projects.  It's good practice to write your own comments.

`users` and `items` are not generic terms.

If you want you can write:

```
For the particular case of Netflix ratings:

* `A` is the "ratings" matrix: it maps from a user and item to a rating
* `U` and `M` are the factor matrices, which represent user and item preferences.

```