The PSR-S series can only use DSP1 with the style parts.
In 'Insertion' mode, DSP1 can be targeted at a single style part. In 'System' mode, DSP1 applies to all style parts but you select how much (0-127) of each style part goes to the effect.
Tyros can use 3 DSP with the style, only Genos can apply a different DSP to each part.