On our assets, we have related opportunities. We want to have a tickbox that is ticked if the stage of the opportunity is invoiced (or negotiation and invoiced). How/Where can we create this rule?
Since each Asset will have a single Opportunity (you can't define a Master-Detail between Assets and Opps), use @Sunil's idea and create an Asset custom formula field of type 'checkbox' (Setup->Customize->Assets->Fields)
