While STM32 VDD is generally supplied from a 1.8V-3.3V source, only a small portion of the chip actually works at this voltage (mainly GPIO). Most of the chip's logic - i.e. processor, bus fabric, peripherals - runs at a significantly smaller voltage (sometimes marked as VCORE), 1.2V and below. This both decreases power consumption through leakages, and allows to use smaller CMOS transistors geometry. This voltage is generated by an on-board linear power supply regulator (LDO).
Being linear, around a third to a half of the energy consumed in Run mode is turned into heat in this regulator. This may be a concern especially in low-power devices.
To circumvent this problem, ST started to offer some of the newer STM32 models also in a variant, where the internal voltage is exposed to a pin, so the internal LDO can be replaced by a high-efficiency external switched-mode power supply (SMPS). These variants are marked with suffix P.
In some even newer STM32, ST even started to include the SMPS onto the chip, so that only an external inductor and bulk capacitor needs to be connected to have a highly power efficient solution. Variants with on-chip SMPS are marked with suffix Q.
Unfortunately, these modifications mean, that some of the signal pins had to be given up to allow these new power-related pins. But what's even more annoying is, that ST decided also to shift some of the remaining signals to the neighbouring pins, making design of compatible boards harder. The following figure illustrates this problem on the STM32L552 in LQFP64 package, which is manufactured in all three variants, i.e. STM32L552RxTx, STM32L552RxTxP, STM32L552RxTxQ:
As the suffix is located relatively deeply in the parts' marking, it's easily overlooked and may lead to unpleasant surprises, especially if the user is not aware of the problem, and especially if the power supply chain is stressed and depleted, as it was the case in the last couple of years.