They relate to the change in use of properties. Currently, for example, if you own a rental property and you make it your principal residence, there is a deemed disposition of the rental property. The value that it increased while it was a rental property is taxable, and the value that it increased while it was your principal residence is eligible for the principal residence exemption.
Currently there is a deferral available when stand-alone property is changed from, say, rental to personal use, possibly qualifying for the principal residence exemption. However, that election is not currently available in respect of changes of use of multi-unit residential properties. If you have a duplex, for example, and you're renting out both units but decide to change one into your principal residence, that election to defer the accrued gain is currently not available. It's sort of a technical hole in the rules. There's no reason it should be available for a stand-alone property and not for a semi-detached or a duplex.
This would help align the rules for multi-unit properties with those that currently exist for single-unit properties, and it would provide for a more coherent set of tax rules.