...
In the Entity field, select the legal entity of your company for which this document is being prepared. The Entity field is available if the Multi-Company Accounting option is selected on the Administration > Settings > General Settings form. The default entity for all users can be selected on the General Settings form. Also, you can specify your personal default entity, for details refer to Personal Settings.
The Basis field is filled populated automatically, if the invoice is based on a document of the following types: Lead, Quotation, Invoice Received, or Customer Order. If needed, you can manually select the basis document from the list of customer documents (for this, click Show all from the drop-down list). Once the basis document is selected, the data is copied to this invoice. If, after you edited the invoice, you want to cancel the edits, click the Fill in button – the details from the basis document will be copied to the invoice.
...
The Project field is available if the Project Management functionality is turned on on the Administration > Settings > General Settings form. You You can link the invoice with a project from the list of available projects. If the items in this invoice are associated with different projects and you need to specify different projects for different items, you can move the Project field from the Main tab as a column to the table on the Inventory and Services tab. For this, use the Settings button on the toolbar (for more information, see Document Toolbar).
...