"When any positive number other than 0 is entered, it will protect you from contributing multiple times to the same window with the same amount. This is to protect you in the case of network congestion, and pending transactions. For most people 0 is adequate."

From the EOS website:

"Use BuyWithLimit function in the EOS Distribution Contract
If you would like to purchase EOS Tokens in a future period, you can use the BuyWithLimit function in the EOS Distribution Contract and specify which period you wish to purchase EOS Tokens. If it is a BuyWithLimit transaction and you have specifically indicated that the transaction will be terminated if a certain amount of ETH has already been received for such period, then your transaction will be cancelled."

I would like to limit the amount of ETH I spend on each EOS token, is this possible as suggested on the site? I have dug deeper and it seems its only possible at the time you contribute which really doesn't help much if at all as more ETH can be added afterwards without any change to your particular contract.