Patient Number
The patient is the central object of Patient Management and is defined by means of a unique identification number - the patient number.
The patient number can be assigned by the system using internal number assignment. You can also assign the patient number using external number assignment. In Customizing for IS-H, you can specify whether one of the two number assignments should not be supported. In Customizing, you can also define the number range authorized for internal and external number assignment. For more information, see the IS-H Implementation Guide under 
SAP Healthcare - Industry-Specific Components for Hospitals 
Basic Settings Maintain Technical Number Ranges 
.
The system ensures that a unique number is always assigned to the case. With internal number assignment, the system always selects the next number from the internal number range. With external number assignment, the system prevents a number, which is already assigned, from being assigned again.
Internal Number Assignment
With internal number assignment, numbers from a number range defined in Customizing are assigned to the patient consecutively. These are automatically assigned by the system when you create a new patient master record, regardless of the other patient data such as the patient’s date of birth. Consequently, the patient number is non-mnemonic; in other words, it simply provides a unique means of identifying the patient.
External Number Assignment
With external number assignment, the user manually assigns numeric or alphanumeric characters from a number range defined in Customizing.
Check Digit
You can choose to add a check digit to the patient number. A check digit describes the preceding patient number and is used in connection with a check algorithm in order to prevent incorrect entries, for example an incorrect sequence of digits. The check digit is simply a mechanism for verifying input. It is not part of the patient number and plays no part in further patient-related processing. The check digit is generated from the patient number according to the check digit procedure defined by ISO standard 7064.
In Customizing for IS-H, you can either activate or deactivate the check digit procedure for assigning the patient number individually for each institution.
For more information, see the IS-H Implementation Guide under SAP Healthcare - Industry-Specific Components for Hospitals Basic Settings System Parameters Maintain Institution-Specific Basic Control .