Nuna now has two infant car seat options that can be installed without the base using the vehicle's seat belt: the PIPA and PIPA RX. You cannot install the Nuna PIPA Lite, PIPA Lite R, or PIPA Lite LX without the base due to their lightweight design.
The Nuna PIPA and PIPA RX both use what is called the European belt path installation, where you will route the lap belt across baby's legs with the side belt paths, and the shoulder belt around back through the blue shoulder belt guide:
This is a helpful feature if you frequently fly or travel by taxi, since you can leave the base at home if needed and still safely secure the car seat in the vehicle or airplane! However, all infant car seats come with a base that will stay in your own vehicle, saving time and effort by allowing you to easily click the car seat in and out.
Here you can read more about the differences between the Nuna PIPA and PIPA RX, as well as the PIPA Lite, PIPA Lite R, and PIPA Lite LX. If you're comparing the PIPA base with the new RELX base included with the PIPA RX and PIPA Lite R, this blog post goes into more detail between these two bases.