If multiple transactions try to modify the same path, only the last one is valid. For example, if tx1 is {a/b: 1} and tx2 is {a/b: 2}, the last value 2 is the one that is valid. This applies to the read_concern as well. If tx1 is {a/b: 1, read_concern: 1} and tx2 is {a/b: 2, read_concern: 2}, a/b path is not recommended to be read until 2 block confirmations.