Can you tell us about the 14 day rule?

The IRS says in this document:

If you rent a dwelling unit to others that you also use as a residence, limitations may apply to the rental expenses you can deduct. You're considered to use a dwelling unit as a residence if you use it for personal purposes during the tax year for a number of days that’s more than the greater of:

1. 14 days, or
2. 10% of the total days you rent it to others at a fair rental price.

Please read the entire document for more details.

There are several 14 days rules.  Can you clarify what you are referring to?