There are a number of places on the body where these magnetic yin fields can arise in addition to the Dan tians (e.g. perineum, yellow court, etc) Once there is sufficient opening and connectivity in the body just the awareness touching these areas will trigger their fields. these areas can be used in consort in cultivation to work with and build  qi holistically across the whole body if it is open enough and the mind is sufficiently absorbed into the body. There is even connectivity generated to areas outside these fields allowing one to affect the yin fields and surrounding physical tissues  by subtle mental or physical adjustments to body areas far away from them ( or even from points outside the body). The size and shape of points, channels and fields  and  sensitivity to them can also change depending on  where one is on their path and on which path.  All this change/variability/connectivity (while being very Daoist ) can be confusing and a pain for people trying to map out and hold to a fixed view of the subtle body. My advice  is listen to your body, then listen to the teachers and texts, then listen you your body again, then laugh at yourself for overthinking it, let it go and practice some more. 
  the ldt itself is congenitally energetically connected to points in each direction that allow it to be used as an engine for cultivation. While these connections directly relate it to the whole abdomen, I  think one typically has to reestablish the connections to these specific points before it can be used this way.