Help with ALV sorting!!!

somebody can tell me, what are, g_it_wa_sort-spos AND g_it_wa_sort-group for?

I need to do a special sorting.

Check this sample. If i have

FIELD1 INVES-002

FIELD1 INVES-002

FIELD2 INVES-002

FIELD2 INVES-002

and i use g_it_wa_sort I get in my ALV

-

FIELD1 INVES-002

(EMPTY ROW)

-

FIELD2

(EMPTY ROW)

-

but what i need is:

-

FIELD1 INVES-002

(EMPTY ROW)

-

FIELD2 INVES-002

(EMPTY ROW)

-

Because, field has changed from FIELD1 to FIELD2

Can anybody help me please?