The value of the data itself may be so revealing or confidential that it becomes sensitive—for example, a person’s salary or that a patient has HIV/AIDS.
The source of the data may indicate a need for secrecy—for example, an informer whose identity must be kept secret.
The owner of the data may have explicitly declared it as sensitive.
The particular attribute or record may have been declared sensitive—for example, the salary attribute of an employee or the salary history record in a personnel database.
Some data may not be sensitive by itself but will become sensitive in the presence of some other data—for example, the exact latitude and longitude information for a location where some previously recorded event happened that was later deemed sensitive.