If you look at BMW's current marketing names for the i3 and the i8, it describes the i3, both with and without the REx, as "electric", whereas the i8 is a "plug

If you look at BMW's current marketing names for the i3 and the i8, it describes the i3, both with and without the REx, as "electric", whereas the i8 is a "plug-in hybrid", and does have a hybrid powertrain. Obviously it can be argued that the i3 with the REx is a hybrid, but it's clear to me that BMW intends to build an i8-like 3 series.