 | Warning The information in this article is only a draft. It's in a very early stage of writing or not written at all. In short: don't read it if you are not an editor.  |
Problem package
To describe a problem, you should state it (prepare human-readable description of the task), prepare tests (pairs consiting of program input and required output) the users' submissions will be automatically checked on etc. That's what task packages are for. For more information, read Preparing Task Packages document.
A package is only a zip file, therefore it can be reused - you can use it in multiple contests and/or multiple rounds of the same contest.
Problem instance
There is a huge difference between a problem instance and a problem package: problem instance is created when you bind (upload) a problem package to given round. After creating a problem instance, there will be no way to add/remove tests, change statement or modify checker programs. When you want the participants to solve a task during given round of the contest, you should upload it.