It is when you have more than one H-1B employer at a time. Can be multiple, unlimited number of employers, in any variation possible: for example, one H-1B for full time work and another ("concurrently" - at the same time) on a part time basis, or multiple, concurrent H-1B employers, all part time. An important and often time consuming part of the process of obtaining U.S. lawful permanent residence (a green card) based on employment is when the employer files the required Labor Certification (LC) with the U.S. Department of Labor (DOL).