USER LIBRARY option

Hi

Need some inputs on these two options:

I tried two options:

1) Using "set_global_assignment -name USER_LIBRARIES" option (with proper order and only the required files inside the library, so no question of ambiguity) I generated a bit file.

2) Using "set_global_assignment -name VERILOG_FILE" option, I explicitly gave the files.

Using 2, I get an error "part-select direction is opposite from prefix index direction" - I understand what this means, but I dont get this error while using option 1.

Can anyone explain the difference from tool flow perspective between the two options? And why is the error not seen in the first option?

I am of the opinion that, option 2 is better, as there is no ambiguity in the files used. Am I right?

Thanks,

Satish