Exchange Online indexes the items stored in mailboxes. Some of the items are partially indexed, meaning that not all of their content is indexable. Microsoft has a PowerShell script to analyze the number of partially indexed items found in mailboxes. The output is kind of esoteric, so we worked it over to create something more understandable.
https://office365itpros.com/2020/12/04/understanding-partially-indexed-items/
