MES is an acronym used by IBM which stands for Miscellaneous Equipment Specification.
- Any server hardware change, which can be an addition, improvement, removal, or any combination of these. The serial number of the server does not change. Specific types include the following:
- Customer-installable feature (CIF) miscellaneous equipment specification (MES)
- Install-by-IBM(R) (IBI) MES
- Return-Parts MES (RPMES) is a special MES that is an IBI MES and requires the return of selected parts to IBM on completion of the MES. 
Since the MES process involves replacing large parts such as CPU or feature cards, or even the entire server itself, but without a system serial number change, it is sometimes referred to as a Machine Equipment Swap.