When the Linear sources item is checked in the Dose calc. options menu, or the Linear indicator in the Prescription window toolbar is enabled, the dose calculation considers the radionuclide to be linearly and uniformly distributed along the axis of the seed. If disabled, the dose calculation treats the radionuclide as a point source at the center of the seed. Linear sources are normally enabled for plaques with slots and are not applicable for BEBIG Ru106 plaques which use the surface patch source beta emitter model. Selecting the COMS calculation mode disables linear sources, although you could override this by manually reselecting linear sources.
For example, when Linear sources are enabled, the 6711 I-125 seed is modeled as if it were a linear source with a 3 mm active length located along the central axis of the seed. The IAI-125A seed is also modeled as if it were a linear source with a 3 mm active length located along the central axis of the seed.