REDCap is a system for collecting and storing data, including Patient Health Information (PHI) that is sensitive and identifiable. To help maintain confidentiality, REDCap assigns each participant's data (a record) a unique identifier that is not otherwise linked to a participant's personal data. That identifier is know as either a Record ID. The Record ID field is a unique field that is always the first field in the project and is mostly not modifiable. Users can rename the Record ID field, so some users call it a Study ID or something else. REDCap can create this Record ID for you or you can create it manually.
The Record ID field is always the first field in the first instrument listed in the Online Designer. In the screenshot below the Record ID field would be the first field in the Basic Demography Form. But if any other instrument were moved to the first position, the Record ID Field would move to that instrument instead automatically.
Users are allowed to edit the Record ID field to a limited extent, unlike other fields which are more customizable. Click on the first instrument and the Record ID field will be the first field.
Click on the pencil icon to edit the field: 
When you are in the editing field window, you can edit the Field Label, the Variable Name, add validation, and note if the field is an identifier. The Record ID field is normally not an identifier, but users can create their own Record IDs and could potentially use something identifiable, like an MRN or date of birth, but that is not normally advisable. You can find more information about creating and editing fields in the section of this guide 'Online Designer: Instruments and Fields'
Users can disable or enable the system automatically generating Record IDs in the Project Setup screen under 'Enable optional modules and customizations'. Click the button next to 'Auto-numbering for records' to enable or disable the feature. If enabled, the system will create the Record ID when a record is created, if disabled the user will need to enter a value when creating a record.
