'fo.' stands for form (English) / forma (Latin), a lower rank than variety; it should really be abbreviated as 'f.', the standard abbreviation for forma. Just as a species can have several varieties, a variety can have several forms.

"yez." is an abbreviation for a name (a common, but very bad practice, on the part of the nursery); what it is an abbreviation for, I don't know, but the nursery is making the assumption that everyone does know. If it is from Asia, there's a possibility it might be short for yezoensis (from Yezo, an old name for Hokkaido, in Japan), but it could stand for any of several other names, no way of predicting (which is why it is very bad practice on the part of the nursery).

Resin

Pineresin offered a very authoritative reply to the query. I would like also to add that almost anyone today may present himself as a taxonomist and consequently much completely erroneous information is seen via the internet. Two sites that are often useful are Tropicos and ITIS. Plant taxonomy is currently in a state of massive revision... triple check all plant names before you accept them!

Resin nailed it - the correct taxonony on this plant is Aster ageratoides var.ovatus f. yezoensis.

Here is a link that might be useful: linked plant

Thanks a lot. I searched the aster with the complete name on he internet and found some interesting things, mostly japanese stuff about native Asters on lime stone soil. But with some luck a friend of mine can translate it for me.

And egged on by your answers I dug out some books back from university, and in one I found the following (by an German author who refers to International rules for naming stuff)
under the species
you have the subspecies (ssp.)
the variety (varietas or var.)
and finally the form (f.).

And just out of curiosity I will try to digg out why or how so many different strains of a species evolve or whether it is typical for certain types/genera of plants.

Well, thanks a lot, cheers, Lin

ps: and I got the cultivar Asran or Ashran and in a very detailed nursery cataloge it was rated as a real spreader which can hold its own and and is likely to take over a border in he long run. I noticed the runners/rhizomes (ca. 3 inches) when I transplanted some after their first season.