What are the keywords associated with DCB? How can you specify DCB information? What is the OS precedence for obtaining that DCB information, ie. where does the system look for it first?

The OS method for obtaining DCB information--there was none. It was up to whoever was creatingf the JCL to code these parms correctly. If the pgm included some of these; then jcl & program had to agree; otherwise, an S013 abend was received.

There is what is called a merge. The precedence is what is in the program, then what is in the JCL, lastly is what is in the DSCB associated with the file. You may get an open error if the program specifies it and it is not what the file actually is. (Conflicting parms do not cause the abend, only if the overriding one is not correct.)