For some bizarre reason, Apple still signs an old iOS 11.3 Beta, the 6th, and last iOS 11.3 Beta. Of course, iOS 11.3 has left the beta stage months ago and it is no longer signed even as a released and it has been long replaced by iOS 11.3.1 and then by iOS 11.4. At this point in time, Apple would have absolutely no reason to keep iOS 11.3 Beta 6 (which is the same thing as iOS 11.3 release) signed. Yet they do. iOS 11.3 Beta 6 is signed and anybody can get the IPSW and restore to iOS 11.3 from any iOS version.
In one of my previous videos that will be linked below, I have shown you how to downgrade to iOS 11.3 without SHSH2 blobs, however, this bug will not last forever and even tho it is a goldmine for the iOS Jailbreak because it means we can still downgrade/upgrade to a jailbreakable firmware, that won't stop it from becoming unsigned and useless. When that happens, the only way you can install that is using SHSH2 blobs that can only be saved while an iOS version is signed by Apple. If you missed the signing window for iOS 11.3 and iOS 11.3.1 and did not save your blobs for those, it would probably be a great idea to at least save your SHSH2 blobs for iOS 11.3 Beta 6.
Why would you save these?
Well, if you have the SHSH2 blobs saved and the SEP and Baseband of the currently signed version are compatible, you can downgrade to that version for which you saved the blobs at any time regardless of whether Apple signs it or not. The blobs are requested by the phone during restore from the Apple server. If Apple signs the iOS version in case, the server returns the SHSH2 blobs and the phone installs the iOS version. If it is no longer signed, no SHSH2 blobs are generated. If you have them while Apple generates them, you can make the phone think it is stills signed.