Update SC4-1-1-unique-id

[annethyme]

Test case descriptions added + a note for the Applicability

Closes issue: #228

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[WilcoFiers]

@carlosapaduarte awaiting your review for this.

[carlosapaduarte]

Use the plural of id (ids) coherently throughout the test cases (in passed example 2 the plural is used, but not in failed examples 1 and 2)