The New “Reindex List” and “Reindex document library” option in SharePoint 2013
If you have not noticed yet a new option was added to Advanced setting of each List and Library to force the Content of list and library to be re-indexed.
In clear words the “Reindex” option marks the whole list or library to be picked up by a Continuous crawl, regardless of whether the items have actually changed.
How to Reindex a List or Library -
1. Navigate to the List or Library which you need to mark to be re-Indexed. Click on Site Settings -> Advanced Settings
2. Next scroll down and find the “Reindex Document Library” or “Reindex List” button.
3. Once you click on the button a Confirmation window appears.
You need to select “Reindex Document Library” to Initiate the re-index process.
Where is this Useful - This is highly useful if you have configured a new Managed Property and added it as a Site Column to your List or Library.
If you have not set Continuous Crawls the List\Library will be Re-Indexed when the Full crawl kicks in.