There are three types of ID described below.
Global ID: This is a hash/ID assigned by Ripple it is unique to each participant and is global. It is reported in the registry and in every study, the participant is added to. This field is locked and users cannot change it in the Ripple UI. You can change this by uploading participants and including the variables: new Global ID, First Name, Last Name, and Birthdate. Ripple uses these variables to match and update participants and to update you need 3 of these 4 variables.
This would auto-populate and be unique to each individual...you could just copy this when the participant is created and use it as your Custom ID. We can talk about different ways to do this depending on the process.
Custom ID: This is a study-specific alphanumeric ID, the limit may be 12ish characters (I'll find out the exact limit for you). You can have the same Custom ID in multiple studies, i.e. Custom ID 100 in Study 1 and Custom ID 100 in Study 2. However, Ripple will warn you if you repeat a Custom ID in the same study.
Family ID: This is an ID that connects participants together. When participants have the same Family ID they will show up the "Family" section of each others profile cards and you will be able to see the studies they are in, their name, gender, and current age.