This tag should only be used when a key objective in the question relates to any combination of: efficient processes, sustainable architecture, Joomla! Coding Standards, general web security, direct/purposeful actions, and other such foci where optimisation is valuable. This tag is most appropriate when a basic solution is already realised, but refinement is sought.
Best practices should be employed in all design and development endeavours. In the industry of IT, there are often many ways to reach a satisfactory outcome. Determining the best course of action is not always black & white.
In terms of programming, some key subjects are often scrutinised in pursuit of "best practices". The priority of these subjects will vary depending on what is most vital for a given task/project.
A non-exhaustive list in no particular order:
Joomla! has several online resources advising best practices: