Flowering Groups

I'm reading up on fruit trees in preparation to put my first ones in the ground this coming spring. There is so much to learn! Ploidy, chilling hours, rootstock characteristics. I think I'm catching on, but one thing I'm not sure of is what exactly flowering groups refer to.

Are flowering groups distinguished by a distinct (calendar specific) period when they come into flower, or are the trees sensitive to changes in the temperature, daylight hours, etc? Or something else entirely?

Any other knowledge nuggets for a newbie would be appreciated!

I'm pretty sure flowering groups refer to groups of varieties or species that bloom at the same time. In a broad sense there are the early bloomers, which include most deciduous fruit trees, which bloom early enough to be vulnerable to late freezes and so ameliorating this danger can be part of the design of where to place them; and the later bloomers, mostly fruits originating in warmer climates which bloom at leafing out time or later (mulberry, persimmon, most nut trees are examples), which are thus less sensitive to late frosts. On a finer scale, varieties within a species also bloom at somewhat different times, so much so that the earliest varieties of apple to bloom, for instance, won't be good pollinizers for the latest varieties to bloom.....something to take into account when selecting varieties if cross-pollination is desirable. If you have limited space, for instance for only two trees, you need to be sure they are compatible in every way. It took me a long time to thus determine, and then find sources for, the two chestnuts I ended up planting for this very reason.