Source: http://us-non-immigrants.blogspot.co...-to-track.html
'Hard' vs 'Soft' LUD - How to Use It To Track Your Case With USCIS
If you heard people talking about keeping track of 'Soft' and 'Hard' LUDs on forums and other websites, and wondered what it is, now you can understand what is LUDs and how you can use them to track any activity on your petition that is pending with USCIS. LUD is an abbreviation for 'Last Updated Date'. In order to track any activity on your pending petition, you will first need to register an account with USCIS to check your case status online.
Here is a link to register for USCIS case tracking system. Once you are registered to the system, you will need to add each case you want to track based on receipt number on notice that you had recently received from USCIS. You will see your case as shown below. You can track LUD (Last Updated Date) on your case in column shown with red box below.
You can go into each case to check your status. USCIS allows an individual to setup an email or mobile alert to track case status update.
Hard LUDs
"Hard" LUD (Last Updated Date) is one where you see status update to your case. Example from "Initial Review" to "Decision". Here you know why case was updated; you can see the update status online. You can even get an email or mobile alert, if you had opted for this option. However, the status will only change if the USCIS updates your case from one to other (receipt notice, approval notice, request for evidence (RFE), notice of intent to deny (NOID), card production, or other documents) or on receipt of response to RFE, NOID or request to premium process the case.
Soft LUDs
"Soft LUD" is one where you will not see any change in status or get an email alert from USCIS. These are the internal activities that occur at USCIS. For example your I-485 case has been processed and ready to be approved, but USCIS could not approve it because your priority date is not current. Others like your fingerprints get added in the system, your background check results are received, your case was moved internally from one cabinet to other or moved from one service center to other etc. You can track this by logging into USCIS Case Status system and checking "Last Updated" column in your portfolio. If you know that you expect any update in your case due to FP or background check or your I-485 is about to process, you can track this based on "Soft LUD". But again, this is all based on guesstimate and no one other than USCIS knows what is the real reason behind the update. My personal opinion is that I would not lose my sleep over tracking Soft LUDs. You will never get anything useful from it.
Hope this help.