In Clooz, information that contributes to the formation of a complete source citation for a piece of information is contained in both the document and source levels.
A Source in Clooz is a description of the document itself. It could be a census, book, church record book, marriage certificate, tax record, etc. The Repository is where this item is physically held. In the case of online databases, the database, perhaps listed as a Collection, would be the source. The web site/company publishing this database would be the Repository.
A Document in Clooz is where the contents of the source document are entered. It consists of two parts. The general part where some additional information is contained relating to the source (such as the location it applies to). Secondly, there is then a detailed set of information related to each individual person linked to that document. For a census, everyone in the family might be linked, with the document detail containing the information listed on their line of the census. Typically, it is in this detailed set of information where the page number reference is listed, since different persons might be mentioned on different pages of the same document.
This way of organizing information and source identification (some programs call these type of sources Master Sources) results in the most reuse of source records. Clooz is flexible enough for you to adapt these definitions to your own desired approach.