When the ABI of a system call is changed in the FreeBSD kernel, this is implemented by leaving a compatibility system call using the old ABI at the existing number and allocating a new system call number for the version using the new ABI. As a convenience, when a system call is caught by name (see catch syscall), compatibility system calls are also caught.
For example, FreeBSD 12 introduced a new variant of the
system call and catching the
kevent system call by name catches
(gdb) catch syscall kevent Catchpoint 1 (syscalls 'freebsd11_kevent'  'kevent' ) (gdb)