As Mohana indicates this is a dimension.
A dimension is normally used for reporting purposes for separating income and cost transactions on one or more levels under each general ledger account. This means that you can have your project as a entity that you can report all general ledger transactions for each project code.
You will also find the dimensions used many other places in the system like on sales order, item ledger entries etc.
You define your dimensions under the general ledger setup in your Dynamics NAV,
