From the interface you can only set "power off" as action. The percentages is how much overage you want to allow. Say if your plan has a 5TB bandwidth limit, then 100% means you can use up to 10TB in a single month before your server is powered off. 300% means you can use 20TB of data.
Of course you can also set custom values via API for more flexibility: https://api.99stack.com/docs/v1.4/#operation/server-put-option
Note that additional bandwidth over what's included in your plan cost $0.01/GB/mo and it's dedicated immediately, if you don't have enough balance in your account, this could become an issue, which is why default seems to be set to no overage, or "power off at quota" to avoid additional cost. Support always tells me it's cheaper to just upgrade to a bigger plan with more bandwidth included, this is true in most cases, but always do your own research as there may be exceptions.
