Types of occurrences of the entity type in the document.
Note: this represents the number of instances of an entity
types, not number of mentions of a given entity instance.
Values:
OCCURRENCE_TYPE_UNSPECIFIED (0):
Unspecified occurrence type.
OPTIONAL_ONCE (1):
There will be zero or one instance of this
entity type.
OPTIONAL_MULTIPLE (2):
The entity type will appear zero or multiple
times.
REQUIRED_ONCE (3):
The entity type will only appear exactly
once.
REQUIRED_MULTIPLE (4):
The entity type will appear once or more
times.