Learning - Search for Ignored Assignments

Hello,

Context: a Learning specialist has identified and purged “ignored assignments”. They no longer appear in the plan as errors.
However, this Learning specialist doesn't know why the entry of these assignments generated “ignored assignments” which gave rise to this “error” status (she had never explored “Search for ignored assignments”).
Can someone help me understand what Ignored Assignments are and how they appear, and in which cases this “Search for Ignored Assignments” feature should be used?
And if the solution is indeed to purge them? Is it possible to have automatic or automated purging?

Thank you in advance for your help.