in general, in cse chains are evaluated when using IKS functions like e.g. calculateIKSLinears/Angles, SetKinematicTransformation etc. So everytime when the system must calculate any compensation due to rotated axes for swiveling (Cycle800, PLANE Spatial, G68.2) or 5 axis simultanoues motions, this compensation is based on the current active chain.
In the case of the lower turret movements. These are simple 2 or 3 axis motions where the system does not calculate the compensation. The compensation is covered by the tool length and the local offset only. Until its a simple 2/3 axis movement independent if the machining is on different spindles there is no specific chain required. It could be necessary if the Spindle acts as a positional axis and the lower turret movement is in "TCP" (Traori, M128, ...) mode. In this case you have to define a chain and use them.
For sure we added a few more functions (getKinematiChainPos, getKinematicChainAxis, etc) which using chain information. But these functions returning information about the kinematic model only which can easily reused instaed of defining them reduntant in the mcf/ccf file. So maybe you want to find out the rotary vector of the lower turret axis, for what reason ever. You can define the chain and read out this information for further calculation.
Let me know if you have more questions or would like to talk in more details.