On the Estimate overlay, you can add additional information by clicking the Note icon on the far right of each line item:
This will reveal a dialog with several tabs:
Estimate Desc. – Defaults to the Service-level Invoice description if an Estimate Description is not customized at that level. You might add a more detailed description of what the service includes. To display this to the client, your Estimate Grid must contain the Estimate Description field.
Job Note – This is where you can enter detailed job instructions or click Append / Replace to make the estimate description part of the job note. If you schedule the Job from the Estimate, this information will display in the Job Notes for your Resources.
Invoice Desc. – Defaults to the Invoice description at the Service level. Change, if needed. Optionally, you can click Append / Replace to apply the Estimate Description to the Invoice.
Line Item – Use this optional field for private notes about this line item. Your client will not see this note.