Exactly as described, this holds the actual sequence of up to 16 letters, from a through h and u/v/w/x, defining which frame sizes and in what order this EMIX pattern generates, with each letter mapped to a fixed size such as 64, 128, 256, up through 1518, MTU, or a custom "u" size. The order of letters here, not just which sizes are included, determines the actual traffic pattern a generator produces over time, so two sequences using the identical set of letters in a different order will still stress downstream buffers differently, which matters for tests specifically probing burst or buffer behavior rather than steady-state throughput. For example, a sequence of fifteen 64-byte frames followed by one MTU-size frame creates a very different instantaneous load spike than alternating between the two, even though both use only the same two letters.