The choice of the size (strength) of the quiver tip is influenced by a few different factors:
1. The Casting weight of your chosen method, a weaker tip will not aid your casting with, say, a 3 or 4 ounce loaded method feeder, and could actually break in use. In that case you would need a tip of at least 3 ounces or even more
2. The flow or Undertow, If the tip is dragged right round, by the flow on a river or undertow on a still-water, you will not be able to see the bites. So it is better to select a tip for the flow rather than anything else.
3. The size of your target fish, If you’re targeting shy biting silverfish use a very light tip. For larger species like carp, tench and barbel, you should use a heavier tip. The bites from these species aren’t shy and they will often try to pull the rod in.
Then, to make the topic even more confusing, you have to consider the material that your tip is made from; typically you can get tips made in both Glass and Carbon.
Glass tips have a soft, progressive action and are produced in lower test curves (½ ounce to around 1½ ounces, so are better suited for the more shy biting fish, Roach, Rudd etc.
Carbon tips are stiffer and generally have a higher strength rating (2oz to 6oz) so are better suited to when you are targeting larger fish, Carp, Tench and Bream for example.
If you are uncertain as to the material your quiver tips are made from then remember: they are identified by the colour of the tips base. Carbon tips have a dark base; glass tips have a clear or white base.
By the way, the size or rating of the tip is the pull that is needed to bend the tip through 90°
I hope this helps and doesn't just confuse more . . . . .