The server install image allows you to install Ubuntu permanently on a computer for use as a server. It will not install a graphical user interface.

ubuntu-16.04.6-server-i386.iso - 32-bit PC (i386) server install image
	For almost all PCs. This includes most machines with Intel/AMD/etc type processors and almost all computers that run Microsoft Windows, as well as newer Apple Macintosh systems based on Intel processors.
ubuntu-16.04.7-server-amd64.iso - 64-bit PC (AMD64) server install image
	Choose this if you have a computer based on the AMD64 or EM64T architecture (e.g., Athlon64, Opteron, EM64T Xeon, Core 2). Choose this if you are at all unsure.
ubuntu-16.04.7-server-arm64.iso - 64-bit ARM (ARMv8/AArch64) server install image
	For 64-bit ARMv8 processors and above.