What you get really for tracked change depends on what's in the username field under File|Options|General.

Checkboxes display check characters, not text. A macro could be used to insert the initials after the checkbox (e.g. in a text content control). Besides which, simply recording the initials isn't especially helpful if someone unchecks a checkbox someone else had checked - without the checkbox being present, you'd never know what the final check state was.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]