Every release of the design calculator, what went into it, and what is being weighed for the next one. Buy the workbook once and every version after it is free — your original download link serves the newest build, so there is nothing to repurchase and nothing to chase.
When a new version is published, the download link in your original receipt starts serving it. The link does not expire and does not change, so there is no new purchase, no new link, and nothing to ask me for. If a release is worth knowing about, you also get an email.
The version number and its date are on the Start Here tab of the workbook itself, with the same history you are reading here — so you can always tell which build you have open.
Nothing here is promised, and the order is not fixed. What people actually ask for moves up.