For example, the second input of Copy to Points is dotted:



What does it mean? I googled and found someone says it means unused node. But it makes no sense. The second input of Copy to Points is obviously used. And it doesn't seem to mean "no dependency" either. When I change the upstream node of the second input, Copy to Points gets re-cooked.

it can mean several things, but it highly depends on the node type and intention of the author
it's important to know that the style of the connection is just a visual indicator that any node type can be using differently without any functional difference to the node functionality

when defining HDA you can turn any input into dotted line by checking "Is Reference" in Input/Output tab

which is one of the uses, to mark reference inputs, so inputs that are only used indirectly
like in copy for example, the Target Points from second input are not output directly as a result so you may think of them as a reference for where the first input geo is copied and how
but again this is just a loose concept that author of the node type can follow or interpret however they want, you can define all inputs as dotted lines if you want or none, there is really no functional difference it's merely a visual distinction that can be helpful or confusing depending on how it's used and whether it makes logical sense in any way to have such distinction for any particular node type

while in HDA the definition of dotted/solid connection visualization is static, for HDK nodes is seems to be able to change dynamically
like in Switch SOP for example, where all non-current input connections are visualized as dotted lines, in Switch SOP case it visualizes inactive streams which are not cooked during the execution of the node, in which case such visual distinction is super helpful