The soleus is a flat muscle of the calf which runs beneath the larger gastrocnemius muscle. During the standing calf raise the gastrocnemius is the primary target, with the soleus virtually inactive. However, during the seated calf raise the roles are switched. The gastrocnemius is muted due to its attachment being above the knee, and the soleus becomes the primary recruiter.
The soleus, for most people, will provide less potential for growth and strength compared to the gastrocnemius. Firstly, as mentioned above, the soleus is a thin muscle whereas the gastrocnemius composes the bulk of the calf. But more importantly, the soleus has been shown to have a high abundance of type 1 muscle fibres which have a low potential for growth and force production. The gastrocnemius on the other hand has the opposite composition, with a high concentration of type 2 fibres.