Apple just stopped signing iOS 11.3 firmware, the third major update of iOS 11, so you can only restore your device to iOS 11.3.1 in case facing some performance issues. Also you can no longer downgrade your device to a previous iOS firmware.
Normally, Apple stops signing previous versions of iOS, once a new version is released, is the way that Apple has to make sure that all users are always updated to the latest version of iOS that the company currently offers users, a version which in theory protects from any security problem that has been detected up to that moment.
In addition, iOS 11.3.1 is released to fix small operating bugs, and other problem that some devices have when they changed the screen in an unauthorized establishment.
Currently, the Cupertino guys are working on the next big update of iOS, update that will be by number 11.4. This update if it should offer the function that allows us to synchronize messages through iCloud as well as activate AirPlay 2, a function that allows us to play content independently in each of the speakers or Apple TV that we have spread around our home from a single device.
The release of the final version of iOS 11.4, is likely to be delayed until the end of this month of May or early June, date on which the World Conference for Developers (WWDC) will be held, in which the Cupertino boys will present all the news that will come to Apple’s mobile and desktop products starting in September.