Descriptor state-space models can include both dynamic and algebraic equations, as is common in electrical circuits or constrained mechanical systems. This makes modeling easier and allows for more general systems.
StateSpaceModel — model a linear descriptor system
TransferFunctionModel — model an improper transfer function
StateFeedbackGains — pole placement-based feedback gains
LQEstimatorGains — LQ-based state estimator gains
KroneckerModelDecomposition — decouple fast and slow subsystems