XiaoBaiTu and PJiang, ask question for Bill HR.5634 for PH.D

I read this bill from some website.

H.R.5634. To exempt from numerical limitations any alien who has received a Ph.D. from an institution of higher education within the 3-year period preceding such alien's petition for special immigrant status.


I want to ask what is special immigrant status? Is it new category and what difference between it and EB1a, EB1b and NIW? What I want to ask is if a Ph.D who already applied either above category (Eb1 and NIW) but received his Ph.D more than 3 years when this bill is passed, is he eligible for claiming this special immigrant status? Is this a new employment based category for a US Ph.D to apply within 3 year period after they receive his Ph.D?

Please teach me some knowledge.

Thank you very much!