Pull request system aliases should use the IssuesAliasConstants.MainCakeAliasCategory and PullRequestsAliasConstants.PullRequestSystemCakeAliasCategory constants for defining their category:
[CakeAliasCategory(IssuesAliasConstants.MainCakeAliasCategory)]
public static class MyPullRequestSystemAliases
{
[CakeMethodAlias]
[CakeAliasCategory(PullRequestsAliasConstants.PullRequestSystemCakeAliasCategory)]
public static IPullRequestSystem MyPullRequestSystem(
this ICakeContext context)
{
}
}