URL identifier and URL scheme
According to Apple documentation they don't give much info about the identifier and why it is needed or where it is used. They only say that it should be unique:
A string containing the abstract name of the URL scheme. To ensure uniqueness, it is recommended that you specify a reverse-DNS style of identifier, for example, com.acme.myscheme. The string you specify is also used as a key in your app’s InfoPlist.strings file. The value of the key is the human-readable scheme name.
Also if more than one app specify the same url scheme, then the outcome is unpredictable:
Note: If more than one third-party app registers to handle the same URL scheme, there is currently no process for determining which app will be given that scheme.
The experience taught me that the app who first registered that scheme will be opened, but this may be wrong.