A rental real estate enterprise is a Safe Harbor defined by the IRS specifically with respect to QBI income deduction under 199A. For that purpose it is defined as "an interest in real property held to generate rental or lease income. It may consist of an interest in a single property or interests in multiple properties.) Note that Commercial and Residential cannot be combined within the same enterprise. https://www.irs.gov/newsroom/irs-finalizes-safe-harbor-to-allow-rental-real-estate-to-qualify-as-a-b...
More to your question, if you are choosing to use the Safe Harbor and meet the full qualifications you must assign an enterprise for each property you wish to include. If for example you were combining all your commercial properties into enterprise #1 you would enter a 1 in the RRE# box ( it is basically the same as entering an Aggregation Code.) In fact if you have entered an Aggregation code and now wish to create a RRE you will need to remove it from the 199A worksheet and enter it on the RRE Statement instead. Gotta love the TCJA.
