The info below presupposes that the UK leave the EU and no 'deal' is reached and the UK becomes a Third Party Nation.
In that case going to a different Schengen country will not bypass the 90/180 day rule e.g Spanish visitor going to Portugal.
A multiple-entry visa allows its holder to go in and out of the Schengen Area as many times as he or she wants, as long as they do not violate the 90/180 rule.
Based on how frequently you travel to Schengen zone, you may apply and obtain one of the following multiple-entry visa types:
1 year multiple-entry visa
3 year multiple-entry visa
5 year multiple-entry visa
"The 180-day period keeps rolling. Therefore, anytime you wish to enter the Schengen, you just have to count backwards the last 180 days, and see if you have been present in the Schengen for more than 90 days throughout that period."