A way to classify or categorize documents, items or any transactions in ERPNext is by adding tags "#".
Tag can be added to a DocType just like Assignments and Comments.
To do this:
1. Login to ERPNext.
2. Open a DocType you will put a tag.
3. On the left section of the document, enter the tags. In this example, tags will be added to a lead.
4. Type the tag and press enter.
5. Click Save.
6. Going back to the lead selection page, you will now be able to filter the records by selecting the Tag.