//SYSLIB   DD   DSN=SYS1.MACLIB,DISP=SHR

From this data set, the assembler obtains macro definitions and assembler language statements to be called by the COPY assembler instruction. Each macro definition or sequence of assembler language statements is a separate member in a partitioned data set. The member name is the operation code used to invoke the macro in a macro instruction, or the operand name in a COPY instruction.

The data set can be defined as SYS1.MACLIB, or your private macro definition or COPY library. SYS1.MACLIB contains macro definitions for the system macro instructions provided by IBM. Your private library can be concatenated with SYS1.MACLIB. The two libraries must have the same logical record length (80 bytes), but the blocking factors can be different. The applicable JCL Reference explains the concatenation of data sets.