I've seen this sort of thing being involved with the supply chain. An abbreviation is already taken or the correct one requires a new format. Similar to alpha-numeric codes omitting (alphabet) O. They might have chosen an incorrect three letters over a correct four as a precaution or restriction.Unless it's a misprint. Paperwork says ESL module . I never heard of it either. It may be same thing. Kia may have just renamed it for their vehicle
Do you suppose post HECU is from Canada, post ESL is from US would explain the variance, if that's all it is.