Timer Iterations and memory optimisation

Hi,
In some of the examples three types of iterations have been used viz. -1,1,0
1 and 0 have already been mentioned in the above document, but what is the use of -1 iteration.

Secondly, I am making a game that includes parallaxing and some bullet shots. I want to ask if enterFrame event consumes more memory or timer event consumes more.

Please describe.

Thanks…
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