In newer STM32 families (starting with 'L44), there's an optional internal reference voltage source, VREFBUF3, which can typically provide two output voltages, nominally 2.048V and 2.5V5 (provided VDDA is high enough).
If VREFBUF is enabled, its output is connected directly to VREF+, internally.
In most (although not all) STM32 models in smaller packages (typically below 64 pins), VREF+ is internally bonded to VDDA, to allow for more GPIO pins. In such models, of course, VREFBUF output would clash with VDDA, so it is not allowed to switch it on, effectively making it unavailable.
3. Not to be confused with VREFINT reference source, which is present in all STM32, connected internally to one of the ADC input multiplexer's inputs. VREFINT thus provides only an indirect reference in case that VREF+ voltage is relatively stable but unknown (e.g. coming from a battery).