How important is transmutation circle pattern? It is clear, that transmutation circle in FMA plays significant role for transmutation result. I really doubt, that, for example, Isaac McDougal can use Mustang's circle for water transmutations, and vice versa. But during Brotherhood series, there several example of usage of same transmutation circle for different goals. 

First example - Alphonse circle. For instance, aside of creating things such as cage for Paninya, in Liore he also manages to fix radio (which is fairly complex device) with same circle.

Another example is Roy Mustang, who was able to use his circle for at least two really different transmutations - his common attack with burning oxygen in air, and dissipating water to hydrogen and oxygen during first encounter with Lust under fifth lab.

So, question is - how important is pattern of circle, and is there any hints, what variety of transmutations are allowed with single circle?
Note that Al's alchemy tends to always revolve around the displacement of material. Comparatively, Mustang's alchemy revolves around chemistry (igniting materials) 

Al may be using the same circle which allows him to use a form of "telepathy" (for lack of a better word) to piece things together. 

From many instances of alchemy being used, you can see that the intent of the alchemist changes the effect of the transmutation.


Mustang's glove carries the same symbol, but Mustang has many fire transmutations up his sleeve.
Armstrong's glove is the same (for earth and metal related transmutations) 


This is a recurring pattern for many alchemists. Their circle limits their alchemy to a specific realm (fire, earth, destruction, displacement, ...) but they use the same circle to perform many different transmutations within that realm.

Alchemy seems to be a combination of the circle (method of transmutation => Al's circle displaces material) and the alchemist's intent (how to apply that method => Al puzzles the thing together again).