All of categories are invented to group and sort images published in that site. A definition of a category is not arbitrary but just describe my view on a particular photograph.
Architecture photography. In my point of view it is not just a matter of taking picture of a buildings. First of all a particular construction is for me an emanation of a spirit witch is typical for a district, a city or a group of people who is living nearby. My photography of architecture ought to show not just a walls or a details of a building but to convey social content and essence. For me singular photo is not able to present every feature of its subject. But presenting one photo I barely show my interest and wander how to treat the topic in more intensive way.
Candid portrait photography. Is a photography that visually depict people – their face or posture. In my opinion such a image should be taken without a subject’s knowledge or even a suspicion of being depicted. That conditions cause that a subject is described possibly natural. So the photograph describe person’s clothes, gesture, mimic, way of behaving and being. That crumbs of a visual information says something about person’s frame of mind, character and temperament.