Is the description in Box 14 USCOSR an additional retirement contribution? Or are these wages paid for a leave of absence?

You will need to ask your payroll department for the specific breakout of the acronym since it is not common.  Box 14 is typically used by the employer for informational purposes.  As such they can basically put whatever they want in there.  If the information does not match one of the common codes listed in TurboTax in the Box 14 category dropdown box in the W2 form entry topic, you should select "Other (not classified)".   

NOTE:  Since box 14 is typically informational, data entered there that does not match a specific code will not affect your tax return calculations.