ETC Congo Family v6.4.0 - Manual (Page 388)

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Keep original spacing and gaps will keep preset numbering as it was in the old play, using
the start at value to create an offset. For example, if you drag in presets numbered 1, 1.5
and 3 into the current play with a start at value of 5, the new presets will be numbered 5, 5,5
and 8.
Convert to Consecutive Numbers will clean up decimals and gaps in the preset list - so
dragging in the same three presets 1, 1.5 and 3 into the current play using this setting and a
start at value of 5 will create presets 5, 6 and 7.