For Salesforce, the record Ids (which the Salesforce Org ID is as well) is case sensitive for the 15 character version and case insensitive for the 18 character version. The 18 digits is generally used by consultants for data migration etc.
The first 15 characters is all that is generally needed.
We have provided only the first 15 characters as this matches the display of your Salesforce Org Id as shown in the Company Profile Section.