ISRFIND, ISRDDN and DDLIST are 3 of the most useful utilities under ISPF.
ISRFIND and ISRDDN can be invoked as TSO commands, i.e., by issuing TSO ISRFIND and TSO ISRDDN on the ISPF command line. DDLIST on the other hand is an ISPF command table entry that invokes ISRDDN. This means, that you can call it by simply issuing DDLIST on the ISPF command line (without the TSO prefix).
ISRFIND allows you to view, browse or edit, the data sets allocated to your z/OS ID. ISRFIND can be a valuable tool to ensure that the proper libraries are found in either LPALSTxx or LINKLIST and in the correct order.
ISRDDN is a utility that can help you evaluating and solving problems on your z/OS environment. It provides a list of allocated ddnames, a list of system ENQs, a list of data sets causing system ENQ contention, and a means of viewing storage within a TSO user’s address space. ISRDDN also provides some facilities for gathering information about your system environment.
DDLIST and ISRDDN are synonyms for exactly the same function, which means you can use either one of them.