If your mainframe computer can export its libraries to other computers via the Network File System or a similar facility, the assembler can read macros and copy members directly from the mainframe without requiring a download step. In fact, the object and listing files can also be written directly to the mainframe without requiring a separate upload step. Using NFS can avoid some license restrictions on downloading macros and copy members.
For NFS, the exported libraries are made visible to the workstation computer by mounting the libraries. The host libraries then look like a normal directory to the workstation and the macros and copy members in the libraries look like files within the directory. Once the host libraries are accessible from your workstation, they may be added to the Library Search Path via either the SYSLIB assembler option or the X390LIB environment variable. Usually when the host libraries are exported, there is an option to have automatic translation performed to ASCII when the members are read. For maximum performance this option should not be used since the assembler must translate all ASCII input to EBCDIC for processing.
Although using NFS is probably the easiest way to access mainframe libraries, it may not be the most efficient if the macros and copy members are referenced frequently. Performance may be improved by downloading them to the workstation's disk using the LIBEXIT facility and then having the assembler read them from there.