Many employers and job websites are using a categorical breakdown of veteran status used to determine if a veteran is "protected". Why?

What's the difference between "Special Disabled Veteran" and "Disabled Veteran", and why would being a veteran of Vietnam or recently separated make a difference for OFCCP?

Not sure on the "SDV" and "DV" other than perhaps a 10% disability vs a 30% disability rating. Such as the points for VA/Gov. jobs (5pts or 10pts)
Vietnam Vets fall under their own protected category due to the VEVRAA:
Originally passed in 1974, the Vietnam Era Veterans’ Readjustment Assistance Act (VEVRAA) aimed to provide assistance to returning Vietnam veterans and to protect them from employment discrimination.

It's all about compliance reporting that's established by OFCCP.
Where I work we also award Veterans Preference as established by our state code, Iowa Code, Chapter 35.
Which not all Veteran's would qualify based on when they served

Also Another potential reason why an employer would inquire about your Veteran status is for Work Opportunity Tax Credits.
https://www.doleta.gov/business/incentives/opptax/eligible.cfm

Veterans Target Groups
A new hire qualifies for a veterans target group, if the individual is:
A veteran who is a member of a family that received SNAP benefits (food stamps) for at least a 3-month period during the 15-month period ending on the hiring date;
OR
A disabled veteran entitled to compensation for a service-connected disability, who has been:
Hired within 1 year of discharge or release from active duty, OR
Unemployed for at least 6 months in the year ending on the hiring date;
OR
A veteran who has been unemployed for:
At least 4 weeks in the year ending on the hiring date; OR
At least 6 months in the year ending on the hiring date.
Please note that to have veteran's status to be eligible for WOTC, an individual must:
Have served on active duty (not including training) in the U.S. Armed Forces for more than 180 days, OR have been discharged or released from active duty for a service-connected disability; AND
Not have a period of active duty (not including training) of more than 90 days that ended during the 60-day period ending on the hiring date.

SFC Adam Potter - Thank you Adam.