There are three types of ID described below.
Global ID: Global ID is a hash/ID assigned by Ripple that is unique to each participant and is global throughout all of your studies. It is reported in the registry and in every study that the participant is added to in the participant profile page. This field is locked and can not be changed once assigned through your registry. However, you can change this by updating participants through the import tool and including the variables: new Global ID, First Name, Last Name, and Birthdate. This will match the imported participant to the existing participant using the First Name/Last Name/Birthdate and will then update the Global ID to your preferred format.
Custom ID: Custom ID is a study-specific alphanumeric ID to identify participants individually between each study beyond the Global ID. This may be to number them in order of entering study (1, 2, 3), ordering them in specific groups for the study (1001, 1002, 2001, 2002), or for any other need your study has. You can have the same Custom ID in multiple studies, i.e. a participant with Custom ID 100 in Study 1 can have Custom ID 100 in Study 2. However, Ripple will warn you if you repeat a Custom ID in the same study.
Family ID: Family ID is an ID that connects participants together on participant profile pages. When participants have the same Family ID they will show up the "Family" section of each others profile pages and you will be able to see the studies they are in, their name, gender, and current age.