【求助】adina-f master 命令tractb什么意思?

苍山负雪

TRACTB            [DEFAULT]
Chooses whether or not to use the physical traction boundary condition in the finite element formulation. See the Theory and Modeling Guide.
  
NO    Physical traction boundary condition is not used.
YES  Physical traction boundary condition is used.
DEFAULT = NO When neither normal traction nor concentrated load is applied.
              = YES When normal traction or concentrated load is applied, or when there is a moving boundary.
  
这个physical traction boundary是怎么回事？用不用有什么区别呢？