This is one single-character code (from the fixed set a through h, u, v, w, x) used as a building block for constructing an EMIX frame-size-mix sequence, similar to how IMIX test profiles are described by a string of letters. An admin builds EMIX sequences by stringing these characters together, where each character maps to a specific frame size defined in the companion FrameSize entry, so understanding this character set is a prerequisite to authoring a custom mixed-traffic test profile. For example, an EMIX sequence string like 'hhhu' would tell the generator to send three MTU-sized frames followed by one custom-sized frame, based on what 'h' and 'u' resolve to in this table.