Answer: URN(Uniform Resource Name) is the basically a subset of the Uniform Resource identifier(URL). It uses "urn"scheme for the working an implementation. Uniform resource name functions by identifying components uniquely through their name.
The most common example of URN is ISBN (International Standard Book Number)number which helps in the unique searching and identification of book.
When a formula produces output that is too lengthy to fit in the spreadsheet cell, the error that will show is "#####". When you enter an invalid cell reference in a formula, for instance using "AVE(" instead of "AVERAGE("; the error that will show is "#NAME?". When you type text in cells that accept numeric data, for instance adding 1 + 1 + A; then the error that will show is "#VALUE". Lastly, when you type in a cell reference that does not exist, the error that will show is "#REF".