Allocating Values in Test Modeller does not mean the keys are locked to the application under test. Therefore, if you are in your HR system and Person 987 is allocated to you, there is nothing to stop another developer or tester editing or using Person 987.
The allocated values need to be managed as part of the development team's processes. It is quite common to see teams that have a central spreadsheet where they manually enter keys once they have found them. The Test Modeller allocation, in effect, replaces this spreadsheet.
For automated testing it is much easier as you will just need to run the allocation process just before you run the test automation, see the section 'Running Test Allocation as Part of Your Automation Framework'.
Unique and Non-Unique allocated values. When you set an allocated test to Unique you are saying that returned value is for this test only, the value will not be assigned to ANY other test. If you set a value to Non-Unique, the assigned value could be set in another non unique test or assigned an unused value.
Unique Test: SuiteHuw_TestDeleteCreditCard has Card ID 7 assigned
Non-Unique Test: SuiteHuw _TestReport_1 has Card ID 4 assigned to it
Non-Unique Test: SuiteHuw_TestDashboard_1 has Card ID 4 assigned to it as well
To define uniqueness, Test Modeller needs more information to work out how the returned values are matched with other values. There may be an allocation pool with hundreds of automated and manual tests used for many different parts of the application, you may be using multiple different types of tests (data lookups) but they may well be using the same logical primary keys.