It is possible to attach comments to any cell, this can be done by right-clicking a cell and choosing Insert Comment from menu. An alternative way is by selecting Review – Comments – New Comment.
Comments added in this way can be displayed or hidden by the user, which is done via Review – Comments – and then Show All Comments toggle.
If a user has chosen to hide all comments and comment indicators then any message which has been added to the workbook will most likely go unseen.
However, there is a method which can be used to ensure that a comment will always be seen regardless of whether the user has chosen to hide comments. This is achieved by using Excel’s Data Validation feature, which will display a pop-up message whenever a cell is activated.
To add a message to a cell using the Data Validation feature:
Now whenever that particular cell is activated the message you entered will always appear.