Different hospitals and institutions have different names for some kind of patient identification. Most commonly used is a Unit Record (UR) number. A Medical Record Number (MRN) is also common. In the Patient ID field you can put whatever identifier your institution uses.
In a hospital environment, is is common that wrist or ankle tags contain an identifier. If there is an accompanying barcode, you can use PicSafe to scan the barcode in the Patient ID field.