No set variety of plant makes up this particular category. Architectural plants is a term we at online plants like to use for plants which 'break the mold'
They're just a bit different. Their foliage, color and shape differ from the usual. Generally Architectural plants are one off, an example is the Queensland bottle tree, this looks absolutely stunning as a single specimen plant. However it can equally effective as a pyramid planted group of 3.
Architectural plants do not have to be exclusive and rare, they can be common such as the Silver Birch. Great example of a really underrated tree that stands out for its unique shimmering trunk and foliage. Planted in a group or in isolation can look superb.