This is quite simple. You create your projects in the Dimension tab. Every time you enter a journal entry line for the company you decide if this should belong to a project (dimension). All journal entries belongs to the company but you could define if the lines should be associated with a project (dimension).
This way you can filter the dimensions in most reports belonging to the General Ledger.
Please look elsewhere for a definition of the Dimensions.
/Joe
PS. You can also associate a customer, supplier and/or an item with a Dimension.