In SharePoint Server 2013, there are certain limits that are by design and cannot be exceeded, and other limits that are set to default values that may be changed by the farm administrator. There are also certain limits that are not represented by a configurable value, such as the number of site collections per web application.
Boundaries are absolute limits that cannot be exceeded by design. It is important to understand these limits to ensure that you do not make incorrect assumptions when you design your farm.
An example of a boundary is the 2 GB document size limit; you cannot configure SharePoint Server 2013 to store documents that are larger than 2 GB. This is a built-in absolute value, and cannot be exceeded by design.
Thresholds are those that have a default value that cannot be exceeded unless the value is modified. Thresholds can, in certain circumstances, be exceeded to accommodate variances in your farm design, but it is important to understand that doing this may affect the performance of the farm in addition to the effective value of other limits.
The default value of certain thresholds can only be exceeded up to an absolute maximum value. A good example is the document size limit. By default, the default document size threshold is set to 50MB, but can be changed to support the maximum boundary of 2GB.
Supported limits define the tested value for a given parameter. The default values for these limits were defined by testing, and represent the known limitations of the product. Exceeding supported limits may cause unexpected results, significant decrease in performance, or other harmful effects.
Some supported limits are configurable parameters that are set by default to the recommended value, while other supported limits relate to parameters that are not represented by a configurable value.