Previously this was mixed with vcruntime, with some objects being shared. But this is messy. Instead use a separate library and link ucrtbase to this as well. This also matches more closely how native libraries are organized, where vcstartup is merged into the CRT import libraries, while vcruntime is merged into the static CRT libraries.
vcruntime contains the code that is linked into ucrtbase (in VS it is also provided as vcruntime140.dll)
vcstartup contains the code that is statically linked into executables that link to ucrtbase.dll. In Visual Studio this is part of msvcrt.lib (the import library for msvcrt), similar to our current msvcrtex, and it gets linked when you link to ucrtbase as well. The name is based on the folder name in the library.
Both libraries share some code, but each file is only compiled once.
Timo Kreuzer