For that assessment period, that area will have an assessment, with the attached observation as evidence of it. Whether you want to continue tagging it with other observations is up to you - I used to as I always thought there was no harm in having more evidence for that area.
For the tracking side, the more times you tag it, the more accurate the assessment comes. If you have 1 assessment, then Tapestry will see it as that assessment. If you have 2, it will take the average of both of them. If you're using Refinements, then, for example:
22-36E = 1
22-36D = 2
22-36S = 3
30-50E = 4
So, we have one assessment with the value of 1 (22-36E) and three assessments with the value of 4 (30-50E). To work out the average we would add up all of the assessment values, so 1+4+4+4 and divide this by the number of assessments, which is 4. This gives us a score of 3.25. We would then need to round the number to be whole, so this gives us a score of 3. We can then convert this back to an assessment value, and as shown above, 3 would equal 22-36 secure. Therefore this child's summative assessment, worked out as an average, would be 22-36 secure.
As you can see from this, the more you have, the more accurate it becomes.