When you select Majestic as the primary backlink source (To enable Majestic as the Primary Backlink Source follow this article) you will need to use Majestic Units to recover backlink metrics from Majestic's API.
With Majestic enabled you will see a list of Web Sites, Competitors and External Content that are in the web presence. To enable collection simply check the box beside the URL that you would like to collect data for.
Metric Units - Metric Units are used to collect backlink metrics from Majestic for your domain.
Analysis Units - Analysis Units are used to generate a report of backlinks from Majestic for the selected domains. Note: The number of Analysis Units is different per domain based on the number of backlinks.
Retrieval Units - Retrieval Units are used to retrieve metrics for backlinks from Majestic for the selected domains. Note: 1 unit per backlink.