While in a document, if I make a change in Track Change Options/Advanced Options, the change will occur in all my documents. For example, if I change Deletions from Strikethrough to Hidden the changes is accepted in the open document but is also changed in all my other stored documents. Any ideas on how to unlink the other documents?

There is no way. This is an application-wide setting and is not set in the document. So, if you send the document to someone else, it is their settings that will apply, not the ones you use.

Any idea which setting would allow a change in a single document to affect all other documents?

I sent two documents to someone else. The documents had different "track change" attribute when opened. However, when I made a tracked change (i.e. "strikethrough" to "Underlined") to one document the change also occurred in the other. They remained linked somehow. Does anybody have an idea which setting would cause this issue?

Again, these are application-wide settings, not document settings. They do not relate to any specific document or template, they are how Word works on your computer.

This is like the setting on whether to display field codes or results. It is something that you set for your Word application and it stays set the way you set it for all documents until you change it, even if you close and reopen Word.

It will never change how someone else, viewing the same document on a different computer, sees the document. There is nothing short of complex macros that probably will not run on someone else's machine (and may annoy them if they do) that can attach such a setting to a specific document. Such macros would change the settings on the recipient machine and might well be regarded as acting like a virus by the recipient.

If you want something fixed, save as pdf and transmit the pdf, possibly with the Word document.