Hey there,
I am by no means an expert on lighting and I hope someone with more expertise will update this, but this much I can tell you:
2.0, 5.0, 10.0, etc. indicate the percentage of UVB light that the bulb emits. So for instance for a 5.0 bulb - 5% of it's emitted light is going to be UVB, the rest - regular visible daylight, which is a different wavelength than UVB.
Meanwhile 26W indicates how much power the light-bulb consumes, also the higher the wattage - the brighter the visible light.
From what I know it is generally advised to use 5.0 UVB for animals coming from jungle or rain forest, which would also include chameleons. Meanwhile 10.0 UVB is advised to be used with desert animals, like Bearded dragons. There is some controversy to that however, and some people choose to use 10.0 bulbs for chameleons as well. I use 5.0 for mine and so far it has worked out fine for me. I would say it depend on what size the enclosure is going to be, where you are going to position the light (e.g. how much of the enclosure it will cover and how far it is going to be from the animal) and what kind of bulb - spot or tube are you going to use. Also, you need to remember that UVB bulbs need to be replaced on average every 6 month, because their capacity diminishes over time, and sufficient UVB is very important for chameleons health.
Due to generally low wattage (13W or 26W) UVB bulbs don't emit much heat, so you will also need to install a regular house light bulb for heat/basking. Again, depending on the size of the enclosure and the distance from the basking spot, you might choose anywhere from 40W to 100W. You should have a thermometer to be able to measure both ambient temperatures in the enclosure and temperature in the basking spot in order to adjust the wattage of your regular bulb until you reach the desired temps for your type of chameleon.
I hope this makes it a bit more clear

otherwise you are very welcome to ask questions
Julija