You can either accept the offered class or bypass recall up to TEN YEARS from date of furlough.

Once you bypass they won't call you again but you simply let them know which class you would like (provided classes are available.) So in theory you could take a class a month later or in several years.

The entire time you bypass you are still considered a furloughed pilot and retain your seniority number and travel benefits.

Once every furloughed pilot has been offered recall then UAL can hire off the street. This is why new hire classes the past several years had furloughed pilots mixed in.

I'd strongly recommend that every potentially affected pilot read section 7 of the UPA.