Welcome to the ORM foundation website.
The (.nr) reference refers to the use of a number to uniquely identify a Patient.
Similarly the (.name) reference is used to uniquely identify a Drug.
Another example is (.vin) which is a unique "Vehicle Identification Number") - See >http://en.wikipedia.org/wiki/Vehicle_Identification_Number
One of the big problems in defining requirements (for anything) is that natural language is widely used but it is not suited to this task because natural language is ambiguous and vague.
So, in ORM we uniquely identify things by using references such as (.nr) (.id) and so on.
Unique identification of "things" is one of the core logical principles on which ORM is based. (Google "existential quantifier" for more)
"More detailed documentation" is available in the book "Information Modeling and Relational Databases - Second edition . (see the link on the home page)
However, to make effective use of ORM, there is a great deal to understand. That's why Terry's book is almost 1000 pages. From time to time I run ORM courses that are tailored to meet the specific needs of each client.
Hope this helps