Starting from the iPhone XS, iPhone XS Max and iPhone XR, as well as the Apple Watch Series 4, Apple seems to have changed the format of UDIDs.
While it has been a 40 hexadecimal characters string since the first generations of devices and provisionings, on the 2018's devices it is a 25 hexadecimal characters string with an hyphen.
For example, while an iPhone 8 UDID looks like 123456789abcdef0123456789abcdef012345678, an iPhone XS UDID looks like 00008020-0012345A1234567B.
To find more about UDIDs, you can read the detailed UDID page on the iPhone Wiki.
Both formats work on Apple's developer portal, in provisioning profiles and on Diawi, for installation and device identification using the webapp.
