Meaning of "Computed": This is the default setting for the replacement value. By default the replacement node identify outliers (mean +- 3 SD) for interval inputs and compute this truncated values (Top ceiling) and replaces globally for all inputs. If you want to apply this option only for the selected input variable then you need to click the replacement editor and apply the required setting there.
Instead of using this computed value (Mean +- 3sd) you could also use a user-specified constant value in the global setting or in the replacement editor individually for the selected inputs.
Similarly you can also choose 'Missing'.
In the course demo, we want to replace all zeros only in DemMedIncome variable. Therefore using the replacement editor we selected DemMedianincome and specified 1 as the lower replacement value to identify all the records with 0 income and replaced with missing.
I hope this explanation is clear. If you need further clarification please let me know. Also adding a screenshot of the replacement editor window.