I'm assuming that you aren't a formula office where aux routes are mandatory 6 days assignments. So:
Article 30.2.g.4.
In offices that have not changed the relief days of one
or more regular routes in accordance with Article
9.2.C.5.b., upon accepting an auxiliary route assignment,
the employee must make an election to serve the
auxiliary route six days per week or continue to serve
the auxiliary route and on the assigned regular route
as the primary leave replacement.
a. An employee serving the auxiliary route and serving
on the assigned regular route as the primary
leave replacement may subsequently elect to
serve the auxiliary route six days, if mutually
agreeable between the carrier and the installation
head. Upon this election, Article 30.2.G.1.b. will
apply.
b. An employee serving the auxiliary route six days
may accept a subsequent leave replacement
vacancy in accordance with Article 30.2.E.
5. After completion of the assigned auxiliary route
or the primary route assignment, a leave replacement
assigned to serve the auxiliary route may
be utilized on any other auxiliary route, or serve
as a leave replacement or provide auxiliary assistance
on any regular route.
What that says in a nutshell is management can ALLOW you to chose 6 day assignment, but they don't have too. They can keep it as 5/1.
If they allow you to take the 6/0, then after you finish your aux route they can make you work another route provided the evaluated and actual time do not exceed 12 hours.
Opps. I misread the original post......... I thought the carrier had not made the choice yet. But the choice is made upon acceptance whether to be a 6/0 or 5/1. If 5/1 then what I said about allowing the carrier is correct, management may or may not allow it. But the choice is the carrier's upon acceptance...... not 90 days as poster says.