Most of what the plan computes follows a published rule: a tax bracket, a credit, a contribution limit, a premium. Each of those links to the law or the government page it comes from.
Some things have no published rule to follow. The law may be silent, the guidance may allow more than one reading, or the question may not be about law at all, such as which account to draw from first when cash runs short, or in which month of the year a cost is paid. The plan still needs an answer, so we choose one and say so.
A choice like that is marked Our convention. It is not a rule you must follow, and it is not advice. It is the simplest reasonable treatment we could find, often the one practitioners or the tax authority's examples use, and it is written down so that you can see what the plan assumed.
Where to find them
Every convention is listed on the sources page, in the group for estimates and conventions, with what it says and the page that supports it. A figure that depends on one shows it beside the figure, and the entry here for that topic explains it in words. Many conventions are about the projection itself, such as how the stress tests are sized or how the sensitivity chart steps each assumption.
When a convention changes
If we change a convention, for example because the tax authority publishes guidance, the plan follows the new one the next time it runs. Your inputs do not change.