Okay.. I remember what it was now.. its for users who like to switch back and forth between 3G and 4G devices.. a problem that first arised when the HTC Thunderbolt was introduced.
Here's the info on it:
There are different scenarios with SIM cards; sometimes the card can be used again and other times it would need to be replaced. Below are some scenarios that will help answer this question for you, as well as other users of 4G devices.
If: You already have a 4G device and change your phone number
Then: The SIM card remains active and does not need to be replaced
If: You change from a 4G device to a 3G device
Then: The 4G SIM card retires at midnight.
...original 4G SIM can be re-used with the same MDN (Mobile Directory Number)if the technology change back to 4G is processed prior to 12:00 a.m. ET of the day the 3G change was initially completed.
...After 12:00 a.m. ET, a new 4G SIM is required in order for the line to go back to 4G.
If: You have a 3G device and you change to a 4G device and get a different phone number
Then: The 4G SIM card remains active on existing number when swapped into a different device
...if 4G SIM types are the same, the original 4G SIMs may be used with other 4G Verizon Wireless devices.
...if 4G SIM types are different, the 4G SIMs are not interchangeable. A new 4G SIM for each MDN is required.
If: You port out your service to another
Then: The 4G SIM card retires at 12:00 a.m. ET (Midnight)
...original 4G SIM can be re-used with the MDN if the port out disconnect is reversed prior to 12:00 a.m. ET of the day the port out was initially completed.
...after 12:00 a.m. ET, a new 4G SIM is required.
If: You port in from another carrier
Then: A new Verizon Wireless 4G SIM is always required.