Please see Rajiv's comments on Priority Date Retrogression.
http://forums.immigration.com/entry.php?303-Priority-date-retrogression-consequences-and-concerns
Under the various employment based categories, Category I based permanent residency is the most difficult to obtain. Initially, the foreign national must determine if he/she is eligible for lawful permanent residency according to the USCIS guidelines.
An applicant filing an EB1 alien of extraordinary ability application may self-petition, but an applicant filing under outstanding researcher and/or professor and multinational manager must have a sponsoring employer.
This nonimmigrant classification applies to people who wish to perform services in a specialty occupation, services of exceptional merit and ability relating to a Department of Defense (DOD) cooperative research and development project, or services as a fashion model of distinguished merit or ability.
The H visa category for temporary non-immigrant visas includes several categories: