overflowgid & overflowuid

file: /proc/sys/kernel/overflowgid & overflowuid
variable: kernel.overflowgid & overflowuid
Official reference

if your architecture did not always support 32-bit UIDs (i.e. arm, i386, m68k, sh, and sparc32), a fixed UID and GID will be returned to applications that use the old 16-bit UID/GID system calls, if the actual UID or GID would exceed 65535.

These sysctls allow you to change the value of the fixed UID and GID. The default is 65534.