The distinction between the three generally accepted primary Eldraeic dialects is both informal and quite simple:
“Low Eldraeic” is the language as it is actually spoken day to day, using the common-sense medium of language features and vocabulary that are of use to most of the people most of the time. (It’s still complex and formal by most language’s standards, but it has had most of the rough edges and unnecessary complexity in its native speakers’ eyes rubbed off it.)
“High Eldraeic”, on the other hand, is the language with every idiosyncracy, grammatical feature, additional functionality, and pedantic technical distinction put together by the Conclave of Linguistics and Ontology over generations, for reasons technical, philosophical and political, in play. It is used lightly in scientific and technical documentation where it aids in clarity, brevity and accuracy, more heavily in formal ritual, high-falutin’ rhetoric, and particularly grand opera, and most heavily when one speaker in a conversation wishes to browbeat another about just exactly how much better educated, more intelligent, and generally superior they happen to be.
“Trade”, the third dialect, is the worn-down and bastardized form of the language used widely by non-native speakers who learned it from other non-native speakers, or who found themselves reduced to stammering confusion after taking a mnemonetic course and wondering just how the heck they use all these registers and modes and affixes and non-temporal tenses in practice, and just what is an evidential anyway?