Would Garth “the Gross” Tyrell’s position as Lord Seneschal of Highgarden be as Mace’s treasurer given his consideration as Master of Coin, or more general administrator as a Hand equivalent, or something else?

Garth Tyrell is listed as the Lord Seneschal in the appendix. I was curious as to what exactly that means. And why is house Tyrell the only ones mentioned as having a Lord Seneschal?

Since I got a couple different asks on this subject, I thought I might as well tackle it in one place, as apparently I haven’t before. 

The answer is that the text isn’t very clear about what the office entails. Nor does history eludicate matters, because the term can mean a number of different things:

1. Seneschal can mean “steward” (in the sense of official in charge of administration of the household) although that term doesn’t necessarily mean a high-ranking member of the servant class. 
2. Second, “seneschal” was also an officer of a French (or more specifically Norman and Languedoc) administrative unit known as a seneschalty: seneschals governed the unit on behalf of the king but also acted as a chief justice or bailiff.  

My guess, without much to base it off of, is that when the Tyrells replaced the Gardeners as Lords of Highgarden, they abolished the office of High Steward of Highgarden so as to prevent any comparisons being made between the incumbents and their predecessors. However, the work still needed to be done, so they created a new title for the old office so that they could use it as a cushy job for junior male relations.

As to why the Tyrells are the only ones, I think it’s because everyone else sticks with just stewards and doesn’t see a need to give servants ideas by giving them lofty titles.