Fill parts are parts which are not associated with any solver. Such parts can be used for filling operations, for instance for filling a region into an Euler part using the “Parts -> Fill -> Part Fill” operation (the same fill operation can be also performed with a Lagrange part instead of a Fill part). Fill parts can be also used with Rigid coupling instead of Lagrange parts. Since no deformations and material response get computed during a rigid coupling simulation, it is fine to use Fill parts.