The energy field surrounding the body of both Gen and Sime that is a byproduct of the motion of selyn in the body. Analogous to the Aura that surrounds the body of Ancients

A Gen's nager originates with the production of excess selyn in every living cell.  The Sime's nager originates from the motion of selyn from nerve-cell to nerve-cell (across synapse gaps) and the consumption of selyn in each living cell.  Thus a Sime's "nager" is distinctively different from a Gen's "nager."  A channel (with their Dual Selyn System and selyn-storing nerve networks) has a nager that is more complex.  A channel can "project" a "showfield" that belies the internal condition of the channel, whereas a renSime can not do that. 

A channel can also affect the aggregate of the fields of other people in a vicinity, (the ambient ) to change the way that summation of the nager of every renSime, channel, Gen zlins to others in the vicinity, "managing the fields." 

See Personal Recognizance for progression of a channel learning to zlin other channels.  Also see Channel's Destiny.