I'm not sure what it would "fall into" here, but on live it was discussed on the forums and never said to be illegal. IS boxer, to my knowledge, does not macro anything. It makes switching windows much easier, or you can set up windows of your second (third) box on the same computer to display overlayed your first box. i.e. while playing my Monk full screen windowed I had access to my Shaman and Necromancer spell bars. (worth every penny of $3/month for 3 boxing.)
As described on the live forums IS Boxer does not fall into any illegal software because it takes a live person sitting and actively playing for it to work. No part of playing the game is/was automated. It does not do anything for you beyond making it easier to switch between your multi-boxed subscriptions.
That said, if you're not boxing, I personally see no use for it here. If you want to switch between windows on your computer while playing P99, just put EQ in windowed mode and alt+tab.
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