What are "tagged" Bitcoins? I was reading about the Sheep Marketplace:

Victims of the theft have attempted to identify the thief by sending &quot;tagged&quot; bitcoins to his accounts, using the public nature of bitcoin transactions to follow these moneys through the &quot;blockchain&quot; record of transfers.

What are &quot;tagged&quot; Bitcoins and how do they work? How do they differ from regular Bitcoins?
Source: https://www.theguardian.com/technology/2013/dec/09/recovering-stolen-bitcoin-sheep-marketplace-trading-digital-currency-money
I think it just means that some users discovered this address: 1Eg8WSxvfeZaKmqhNZrECTUb58x3y1AWYN and maybe few others involved in their past transactions to the darknet market. They tried to send small amounts to these addresses hoping they could track when its moved.