This is the system a feature declare its variables in. Typically, this only has an effect if the variable is a spatial tensor (for example, a vector with length 3). It also has an effect for weak form equations, where the base vector system can define the volume factor for the weak form integration. The most common choice is to use the coordinate system represented by the current frame used by the feature. In the Base vector system lists, this is the option Frame system compatible with material type.

The feature determines its frame from the Frame type list, which has the options Material, Spatial, or Selectable by user. The Material option corresponds to the material frame, and the Spatial (typically fluids) option corresponds to the spatial frame. For the Selectable by user option, the frame type depends on user choice or material setting during a Model Builder session.

When you select the base vector system for a feature, it acts as a default for all variables, user inputs, weak form equations, and constraints declared by the feature. If necessary, it is possible to override this default by changing the setting in the Base vector system list under the Advanced section of any of these nodes. Under the same Advanced section for variables, you can also set the tensor type, individual base vector system, and base vector type for each tensor index. In the Tensor type list, choose the type of vector: a Normal tensor, a Tensor density, or a Tensor capacity. Tensor densities and capacities are affected by the scaling of the unit volume during a change of base vector system. For nonscalar quantities, use the Base vector system column in the table to set individual base vector systems for each tensor index. In the Type of base vector column, set the type to Covariant or Contravariant for each index.