I'm fairly new to Ponies so I'll try and help. Foundation is not based on height. A pony can be "Foundation Seal" if in 4 generations the horses do not have a "B" in their registration number or show that the horse is other than Shetland (ie: Americana or Hackney). Classics can have horses with "B" and horses other than Shetland such as the Hackney.
Modern Shetlands have two major height divisions if older than 3 yrs: Over is 43" to 46" and the Under is 43" and Under. 2 Yr olds Overs are 42: but not more than 44.5" and the Unders are 42" and Under. Yearlings Overs are 41" to 43" and Unders are 41" or Under
Classics are as follows:
3 Yr and over: Over - 42" to 46", Unders 42" or under
2 Yr olds: Overs are 41" to 44.5", Unders and 41" and under
Yearlings: Overs are 4)' to 43", Unders are 40" and Under
Foundation Shetlands:
Height is not to exceed 42"
3 Yr Olds and Older: 42" and under
2 Yr olds: 41" and under
Yearling: 40" and Under
Foal of Current Year not to exceed 38"
Shetlands must hav the heel height measured and it can not exceed 1 3/4" including the plate if shod.
No cross entering will be allowed between Classic and Modern classes at the same show.
No cross entering will be allowed between Classic and Foundation classes with the exception of youth halter, color, group halter, and Classic performance classes-unless an equivalent Foundatio Perfornace class is not offered.
I know others will join in and if I have anything wrong I hope they correct me but I did take the measurements out of the rulebook.
I have discovered that ponies are so much fun.