What is an alliance?

[tropterrarium]

Can someone clue me in on the definition of an alliance? Although it is used in the name of many Forum groups, the glossary of Orchidboard does not contain an entry for it! Google? no luck. Wikipedia: nothing. Rather strange.

From what I gather, it is not a formal rank in botanical taxonomy (though Wikipedia implies it a bit). It would be above genus and below subfamily. Supergenus, infrasubfamily?

I only found here (not a terribly reliable source, I fully realize) that "The term Alliance is used to describe a group of genera that have many common characteristics and can be used for cross breeding." So, is it just a horticulturist term with no botanical/scientific underpinnings? Popular grades (as opposed to clades), maybe?

Does that then mean that not all genera are assigned to an alliance? Are alliances just for the favorites amongst collectors and hybrid breeders? Just received the The Marie Selby Botanical Gardens Illustrated Dictionary of Orchid Genera and read through the intro. No word of "alliances", which supports that angle.

Any illumination of the topic would be great. Thanks!