The sunny 16 rule explains that the proper exposure in non-obstructed sunlight traveling through minimal air mass is f/16, and shutter speed set to the reciprocal of ISO sensitivity.

For example:

• If you want to use 1/200 s shutter speed because of the reason that it's the camera's fastest flash sync speed and you don't want to / can't use high speed sync, you need to set f/16 and ISO 200. If you want more background blur, you could move from this one f-stop away to f/11 and ISO 100. If the camera has ISO 100 as its base ISO, you cannot move further to f/8 and ISO 50.

There are several failures of the sunny 16 rule:

• It works only in very narrowly defined conditions. Any clouds? You can't use it. Subject in shadows? You can't use it. Sun not well above the horizon? You can't use it. Those knowledged in photovoltaics know that atmosphere has an air mass effect: light of sun close to horizon has to travel through more air mass than light of sun directly over your head. This air mass effects affects photography equally well.
• It assumes that the camera's ISO sensitivity is accurate. Very often, it is not. If you set ISO 100 and 1/100 s shutter speed, you may get something close to ISO 100 but not quite ISO 100.
• It assumes that the lens has certain transmittance. One lens at f/16 and lots of glass elements (e.g. zoom) may transmit less light than another lens at f/16 and little glass elements (e.g. prime).
• It requires you to do calculations in your head if the aperture you want to use is something else than f/16. Want to use f/8? Set four times faster shutter speed. Want to use f/4? Set sixteen times faster shutter speed.
• If you have a crop sensor camera, f/16 results in more diffraction than optimal. Limiting yourself to f/8 would be far better.
• Even with a full frame camera, an aperture as slow as f/16 can cause dust on the sensor to become more visible than at faster apertures.

Because of the numerous failures, in this era of autoexposure, I would use sunny f/16 rule only as an estimate for correct exposure and estimate for power needed for fill flash in sunlight. Let the camera's autoexposure meter do its job. It probably does the job far better than someone blindly following the sunny f/16 rule.

My experience photographing in sunlight in a northern country where sun is not often very high, is that the conditions where sunny f/16 rule applies are far more rare than the conditions where sunny f/16 rule does not apply.