Question

The principle “What You Specify Is What You Get” guarantees the types of formal arguments of imported functions — an actual argument is guaranteed to be of the type specified for the formal argument, with the exception of open arrays (for which unspecified ranges are statically unknown). Formal arguments, other than open arrays, are fully defined by imported declaration; they shall have ranges of packed or unpacked arrays exactly as specified in the imported declaration. Only the SystemVerilog declaration site of the imported function is relevant for such formal arguments.

Formal arguments defined as open arrays have the size and ranges of the actual argument, i.e., have the ranges of packed or unpacked arrays exactly as that of the actual argument. The unsized ranges of open arrays are determined at a call site; the rest of the type information is specified at the import declaration. See also Annex E.6.1.

So, if a formal argument is declared as bit [15:8] b [], then it is the import declaration which specifies the formal argument is an unpacked array of packed bit array with bounds 15 to 8, while the actual argument used at a particular call site defines the bounds for the unpacked part for that call.