Generally wail tones are deigned to reach larger distances due to larger wavelengths. This makes them much more effective at travelling distances . However, the larger the wavelength the harder it is to project the sound in all directions of the truck as it will mainly travel in the direction the siren is facing.
Yelp on the other hand is a much shorter wavelength and travels all around the truck due to the nature of the sound waves being smaller and therefore creating constructive interference which makes the yelp sound much louder from all directions for example; the side of the truck, compared to the wail which works better in a straight-line at long distances.
Because of that generally wail is designed for alerting moving vehicles on the road which are in front of the truck and yelp is designed for approaching and moving through an area when you need to be heard from all directions such as an intersection where the sound waves would travel efficiently in every direction except behind the truck.
However, normally it's easier to just operate both at the same time in some situations.