IBM 6400 Programmer's Reference Manual page 112

Graphics Command Set
If bit 3 of byte 7 is on, a prologue is the first sequence of drawing orders in a
new segment. The prologue, if present, is always at the beginning of a new
segment's data and ends by an End Prologue order within the same segment.
If bits 5 and 6 of byte 7 are zero, the drawing attributes are re-initialized to the
current default values. If bits 5 and 6 of byte 7 are equal to 11, this segment
appends to the previous segment and the current drawing attributes do not
reinitialize.
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6400 IPDS