When a panel is shaded the voltage will be only a little down on one in full sun, but current will be way down, so any panel in a string that is shaded, limits the current flow in that string to the shaded panel's low output current. Current flow in a circuit must be equal all around that circuit, and in this case will be limited by the weakest panel's output- which is why a chimney shadow can wipe out a grid-connected array's output.
As shown earlier in this thread, having a parallel string in shade doesn't limit the output of one in full sun.
Long strings vs short strings into an MPPT charge controller is a trade-off between CC efficiency and cable losses.
The CC's efficiency will be highest at typically 10-20V above Vbatt (for a 48V system), but the losses in the line
decrease with increasing voltage (ie decreasing current), for a given sized array. Losses go up with the square of the current, I^2 X R, so halve the current to get 1/4 the line loss. If you have a long distance between CC and PV array, then it is generally best to go for the highest allowable voltage, as the decrease in CC inefficiency is only small, but the line losses can be quite large, unless you go to large diameter, expensive cables.
A string refers to a group of panels connected in series, and yes parallel strings is the norm for off-grid, and larger grid connected systems.
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