Hi Priyanka :
Not sure why you are asking a strictly third-party question in an APM group. But I'll bite.
A quick Internet search revealed
etl - Difference between serial file and multi file in AB Initio - Stack Overflow
serial file will contain the data directly.
However a multi file will contain the partitions in which the data will be contained. Once each of the partitions are opened separately then the data in it will be displayed. We use 'cat' for serial files and 'm_cat' for multi files. However if 'cat' is used to open a multi file it will just show the name of the partition files.
Typically the serial and multi file directories will differ in containing a file called .mdir. The .mdir will contain all the partitions that the multi file system will refer to.
Performance wise multi files will increase speed of execution of a graph since the data will be processed in parallel. However in a serial file the same data will processed in sequence.