From MW's article:

Under federal law, troops are barred from performing law enforcement duties in most circumstances, and the order appears to restrict them to a support role unless Mattis authorizes a wider mission.

“National Guard personnel will not perform law enforcement activities or interact with migrants or other persons detained by (Department of Homeland Security) personnel without your approval,” the order drafted for Mattis and signed by him reads.

The order adds that troops will carry weapons only in “circumstances that might require self-defense.”

It’s unclear what operations or missions troops will perform along the border that might require them to carry weapons.

Although barred from interacting with migrants, troops could theoretically be part of joint patrols with Border Patrol officers that could carry risk. Few migrants crossing the border are armed but some patrols focus on interdiction of drug smugglers.

Pentagon spokesperson Dana W. White told reporters Thursday that planners are looking at sending National Guard units that can assist with aviation, engineering, surveillance, communications, vehicle maintenance and logistics support.

Lt. Gen. Kenneth F. McKenzie, Jr., the director of the Pentagon’s Joint Staff, said the military’s preference was for the troops to be armed for self-protection, as National Guard units used along the border have been in the past.

http://www.sfexaminer.com/mattis-aut...border-mexico/
The mission is to seal up the Southern Border per the order of the President of the United States using a military operation. General Mattis is in charge of the military operation to achieve this. I expect them to succeed and I believe they will. I sure hope so anyway.