Hi @Kim_Hakseon,
Partition tolerance is referring to a distributed system continuing to operate even if a network partition or failure interrupts communication between some members. Distributed databases are either CP or AP because they are inherently designed to be resilient. MongoDB production deployments use sharded clusters and replica sets for fault tolerance and data redundancy.
Quoting from my response to a related question on DBA Stack Exchange (mongodb - Mongo CP, Cassandra AP? - Database Administrators Stack Exchange):
CAP is a simplification of real-world behaviour: MongoDB and Cassandra both have tuneable levels of consistency for reads and writes. For example: MongoDB has write concerns to determine the level of acknowledgement required for write operations, read preferences for routing requests to members of a replica set, and read concerns to control the recency, consistency, and isolation properties of data read from replica set and sharded deployments.
Eric Brewer, author of the CAP Theorem, revisited this in 2012 with a more nuanced take: CAP Twelve Years Later: How the “Rules” Have Changed.
Regards,
Stennie