How are non-atomic and atomic properties different? Which one is the default property for synthesized properties? When would a user operate one over the other?

Properties which are stated as atomic are those which always return, with certainty, fully initialized object. The atomic property is also the default specification for all synthesized properties. It is generally good to specify them as atomic in order to completely remove the odds for confusion. However, this guarantee by atomic properties comes at the expense of performance.

But in the case of properties which do not offer risks while retrieving any uninitialized value, setting them to non-atomic properties can actually boost the performance. An example of this is the case where all accesses to the particular property are synced through other means in advance.