Distribution categories show you which categories the document is located in and allow you to move it to a different category by selecting the checkbox next to the category where you would like to place it.
Note that placing the document in multiple categories does not generate multiple copies. The document remains one but becomes accessible from different categories.
To access it click on Settings in the top right corner → Distribution categories → Tick the categories where you would like the document to be and untick to remove it from that category → Save: