Presumably, this question is about OpenStudio.

The space type tab allows you to define different space types, such as "office", "conference room", "corridor", "library", "rest room", etc. Each space type will have a different combination of lighting loads, equipment loads, outdoor air requirements, infiltration, schedule set (occupancy, lighting, equipment, etc.). There should only be a handful of space types in your model, maybe a dozen or less. Think of them as templates, or prototypes.

The space tab allows you to then assign one of these space types to each space you have. For example Room 110 is an "office", Room 235 is a "rest room", Room 480 is a "conference room", etc. You can also do some overrides here, but you can loose sight of the spaces' details quickly if there are many dozens of them. It is better to keep their characteristics organized in the space types, where you only have a small number and can make changes consistently.

Of course, for a highly accurate model, you may need to customize each space, regardless of space type, but there are likely still commonalities among even those spaces that can be maintained more easily via space types. This is really one of the areas where building energy modeling is as much engineering and science as it is art.