Field help (LEVA) 

Level in the sequencing of the documents. Value 1 corresponds to the selected document, value 2 to the document directly linked to the selected document, the maximum value of 15 corresponds to a multi-level log.

For instance:
For an invoice X with an upstream traceability.
Level 1 = invoice level.
Level 2 = receipt level.
Level 3 = order level.

The same level can be used as often as there are levels for this invoice. For instance, there will be a receipt level as often as there are receipts for this invoice.