Names of plants has been an issue forever. Linneaus' attempts to codify all the known plants was a herculean effort, and is the basis of modern Latin naming conventions, and yet it still does not alleviate disagreement on what plant is what. In medieval gardening this is compounded by the passage of several hundred years, and various languages and dialects.
For instance, in the Bountiful Gardens catalog there is a shrub called Serviceberry. Ha! great, thought I. i'll get me some of that! They had service in period, the romans speak of it, Charlemagne listed it in his Capitulare, i wonder what it tastes like...
the serviceberry they are offering is Amelachier alnifolia. Common consensus among garden scholars is that the medieval service was Sorbus domestica. Amelachier alnifolia, also called Saskatoon serviceberry, is native to north america and is a shrub or small tree. Here is the USDA profile for it: http://plants.usda.gov/java/profile?symbol=AMAL2
Sorbus domestica is a tree, up to about 50' tall, and it's fruits are significantly different from the saskatoon. ttp://www.pfaf.org/user/plant.aspx?LatinName=Sorbus+domestica
if you're looking for the service mentioned in the plan of St Gall, then you'd need the Sorbus domestica, pictured here: