1) The "seq# on a tape"... Tapes can contain more than 1 dataset. The "seq#" tells you whether it's the 1st, 2nd, 3rd, etc. The (1,sl... of your example tells you that it's the 1st dataset on a Standard Label (,SL) tape.
2) The VOL=(,RETAIN,99) says, put the DS on any volume(s) up to 99 vols (I think - you can check it) and retain those vol(s) for the extent of the job. You can look at the sysmsg to find the vol sers or you can use listcat or 3.4 of ISPF.
If you used vol=(xxx,yyy,etc...) the system would allocate your DS to those vols.
STEPLIB - Defines a private library that the system makes available for a use by a job step. The steplib statement can appear in any position among the DD statement for the step and can appear in stream procedures. STEPLIB takes precedence over a JOBLIB
STEPCAT -Define a private VSAM or integrated catalog facility user catalog for the duration of the job step. The system searches the private catalog for the dataset before its searches the master catalog or the private catalog associated with the first qualifier of a data set's name.