OK, let me give an example-
Say you are running optimizer in weekly bucket (7 days in a week) and planning start day as beginning of this year, till next 6 months.
So your bucket definition would be like 1-4th Jan, 5-11, 12-18, 19-25 .... like that
Now say in DC1 you have a demand for 100 units of product A on 15th Jan and this DC1 can source from Plant1.
Here the exact availability date to meet the demand is 15th Jan. This length is 4 days from the start of the bucket and the length of the bucket is 7 days. SO the exact availability date for optimizer will be 4/7= 0.57 of the total bucket length.
Now if you have a bucket-offset of, say 0.80, then the rounding limit would be 0.8 * 7= 0.56 which is less than the limit (0.57) for this demand. So the exact availability date falls after the rounding limit, hence the product will be available at the start of the next bucket (at bucket= 19-25) here.
Now if you have bucket-offset of, say 0.90 or bigger number, then the rounding limit would be 0.9 * 7= 0.63 which is greater than (0.57) for this demand. So the exact availability date falls before the rounding limit, hence the product will be available at the start of this bucket (bucket 12-18).
Hope this helps.
Thanks,
Satyajit