Sometimes when using this module, you might come across a search that doesn't return the expected result (or facet, highlighting, etc.). When you've looked at the other sections in this handbook (especially Special server features and pitfalls) and not found anything, looking at the Solr request directly to see where the problem is might be the best next step. (It is also often a great help when you create support requests in the issue queue.) This is usually (depending on your server setup) not particularly complicated.
This plugin checks to see if the current pipeline value is empty (for example the empty string, FALSE, or 0). If so, the further processing of the entire row is skipped and will not be migrated. Useful when combined with a migration process plugin to check if a related item was previously migrated.