In REI Hub, the Portfolio is the top level of the reporting structure. Properties live underneath the Portfolio, and Units live underneath Properties (if applicable). Optionally, you can also enable a Sub-portfolio level of reporting for groups of properties, that lives between the Portfolio and the Property.
Generally speaking, a Portfolio will correspond to a legal entity. Most separate businesses (separate legal entities), need fully separate books, and thus their own Portfolio. This is especially the case when multiple beneficial partners/ owners are involved, as in a partnership or multi-member LLC. If your legal entity is required to file its own tax return, you should most likely be keeping it in its own Portfolio.
The main exception is if all your properties are either in your name, or held in single-member LLCs for asset protection. In this case, there are no other beneficial owners or legal entities with individual filing requirements, so everything is a pass-through that flows through to your personal tax return (on the IRS 1040 Schedule E). So with only one tax return involved, you’re generally in the clear to keep all your properties in just one Portfolio.
This is also an excellent application for Sub-portfolios - you could create one Sub-portfolio for each of the LLCs to easily view aggregate reports.
Each Portfolio carries its own subscription (and 30 day free trial!). You can add and manage multiple Portfolios from one set of REI Hub log-in information. Access for additional users is also segmented at the Portfolio level.