All around the earth there are many climate zones, some very hot & dry or humid, some are extremely cold, or moderate. There are high & low elevations which affect temperatures, wind, and so on. In all those regions different plants and trees have adapted to the 'local' climate.
I can't get all that scientific - and I'm not that smart anyway. But generally an 'outdoor' tree means the trees that grow outdoors in any particular place or region. In that regard all trees and plants are outdoor trees. That's where NATURE put them. That's what is called their 'natural environment'.
Then WE come along and try to move them around. Trees that are native to tropical areas (such as Caribbean Islands, Latin America, regions around the equator) like the ficus family or say a Palm tree will not survive outdoors in the cold north. They are outdoor trees where the weather is to their liking (warm enough) but would have to be grown 'indoors' where it is colder. More simply put northerners would call them indoor trees while people in their native lands would not.
Likewise, in the cold regions, the native trees grow in spite of frigid weather. They adapt by 'shutting down' in winter, storing energy for new Spring growth. Dormancy is the period when they rest up. They need this in their life cycle. It's in their genes - their genetic makeup. SO if we try to bring those tree indoors (when) they belong out in the cold and make 'indoor trees' of them we have taken them out of their native environment and interrupted their normal life cycle.
There are also many tree types with larger native zones, spanning both warm and cold regions - temperate areas. They are more adaptable to being in or out.
But all in all - you just should not expect a tropical banyan to live in the hills of Northern Vermont - or a Canadian hemlock to grow in your living room - or in Miami. Think "native environment" when considering bonsai material. If it would flourish out in your yard year round it definitely belongs out there. But if you want a bonsai for indoors get something that thrives on temperatures in the 60's, lower light levels (than outdoors) and dry air.