In the Gen, "High Field" refers to carrying enough selyn to distort the local ambient nager so that there's more selyn in the Gen than in others in the vicinity. "High Field" can also refer to a Gen's personal state relative to their cycle of production and donation of selyn.
A High Field Gen is noticable to any Sime in the area who is zlinning. A Gen who has given all the selyn the Gen can give without dying is just barely noticable to nearby Simes, and not at all noticable if there is a High Field Gen in the area. Thus the "High" in "High Field" is a very relative term having to do with how noticable the Gen might be.
A channel is "High Field" when the channel's Primary System, which carries their personal supply of selyn, is at full capacity. By using the dual nervous system, Primary and Secondary, to produce an interference pattern in the ambient nager, a channel can create the illusion of being High or Low Field. This interference pattern is referred to as a showfield -- or the field the channel allows to show. Control of the showfield requires a fine degree of mastery of the functions of the vriamic node that connects Primary and Secondary systems, and ample selyn in both systems. The higher order channels have more precise showfield mastery, so that renSimes and lower order channels can't zlin the true state through a deliberately adjusted showfield.
When high field, the individual's Nageric Field is very strongest. As the selyn is depleted, through consumption or donation, the field level diminishes. In the Gen, selyn production immediately begins elevating the Gen's field. (See also: Low Field)