Actions performed with Teamstudio tools are performed under the user's id. Merging requires Designer access to the target database and may be restricted further by Author's fields.
Some items can not be merged because Notes does not allow it in the API. Good examples being table cells and also columns.
Lines of code can not be merged, whole subs/functions can.
Delta does not merge when comparing data.
Delta of 2 Documents does not allow merging.
The Delta window contents can be highlighted and copied to clipboard so values that can't be merged can be copied and then pasted in Designer.