When the Modeling Toolkit is active, transform constraints let you slide components along the edges or the surface of the active mesh.
Note: 
You can set another surface as a constraint using Make Live. Transform constraints are disabled when Make Live is active. See Retopologize a mesh with Quad Draw.
You can set another surface as a constraint using Make Live. Transform constraints are disabled when Make Live is active. See Retopologize a mesh with Quad Draw. - Off
- Disables transform constraints. Lets you use the transform tools to translate, rotate, or scale your components without a constraint.
- Edge Slide
- When one of the transform tools is selected, slides the selected components along the edges of the active mesh.
- Surface Slide
- When one of the transform tools is selected, slides the selected components along the surface of the active mesh.