In SharePoint 2013 and since the release of its RTM, Microsoft has gone through several testing cycles either by the TAP teams, selected partners, etc…, such testing ended up with a list of boundaries, thresholds & limits which generally differs in its nature, flexibility and supportability, to understand better the difference between Boundaries, Thresholds and limits consider the below examples:
Boundaries:
Number of Zones in a web application are 5: Default, Intranet, Extranet, Internet and custom, there is no way to exceed a boundary unless Microsoft changed it in the next SharePoint version
Thresholds:
A document size threshold in SharePoint 2013 is by default set to 50MB however it can be changed to reach the maximum boundary of 2GB which implies that this threshold is the initial recommended value that can be changed “IF REQUIRED” to maximum reach the boundary.
Limits:
Number of Application pools in SharePoint 2013 WFE have a limit of 10, you can theoretically create much more BUT the performance cannot be guaranteed and most importantly Microsoft will not support your environment as you exceeded the SUPPORTED limit.
So Always try to stick to the recommended Thresholds as much as possible, if there are requirements to exceed the threshold it is fine to carefully increase it up to the boundary which is the maximum that you normally should not reach unless it is a must, for supported limits you should never exceed but think about workarounds or different design options.
