Contra means ' a prefix meaning “against,” “opposite,” “opposing” '.
In terms of Sigma, it is cancelling a Bill out against a Credit so that neither appear in the Reporting and do not show as overlapping bills in Bill Entry.
In this example, there is a Bill and then a Credit for the same dates, amount of units and cost.
As the date range is the same, Sigma initially sees this as an overlap. Removing the A and R (Reporting) flags will remove the Bill and Credit from Reporting and remove the Overlap marker.
The lines are now greyed out, the gap doesn't show and neither will be included in Reporting.
If there is a Bill in the database and you then receive a Credit for the same dates, amount of units and cost, you can simply Contra the existing Bill - rather than adding all the details of the Credit.
Check that the Credit is the same as the existing Bill.
Now enter the new Bill for the period.
The Supply Summary will still show the first Bill and Credit, but they are greyed out and not included in Reporting.