The current Packaging Standards include the following clause to address the use of new variables:

"Try to avoid introducing new variables into your PKGBUILD build scripts, as these could possibly conflict with variables used in makepkg itself. If you do find it necessary to include a new variable please prefix the variable name with an underscore"

In the AUR at the moment, we have a growing number of examples of new and custom variables which are avoidable and unnecessary, but the wording above makes it difficult to present a convincing argument. I would like to suggest the following as a possible alternative:

"Do not introduce new variables into your PKGBUILDs, unless the package cannot be built without doing so, as these could possibly conflict with variables used in makepkg itself. If a new variable is required, please prefix the variable name with an underscore"