Difference between slis_selfield and slis_t_extab.

Hi friends,

Enlighten me on this, as to

when am I using this------

FORM user_command USING r_ucomm LIKE sy-ucomm

rs_selfield TYPE slis_selfield.

and when am I using this------

FORM set_pf_status USING rt_extab TYPE slis_t_extab.

SET PF-STATUS 'ZNEWSTATUS'.

My question is .. in what situation am I using the former and the later..

Thanking you all in advance,

Hari Kiran.