Describing collision in sdf

Whats the criteria for defining the ode,bullet and dart solvers ,
referring to the collision tree friction and contact have 2 solvers each
1.should both or only one solver in each node be included
2. If only one of the the two ‘ode’ or ‘bullet’ should it be based on the solver type used in the physics node

Collision
|
surface
| - friction
| | - ode
| | - bullet
| - torsional
| | - ode
| - contact
| | - ode
| | - bullet
| - soft_contact
| | - dart

Thanks