6.0 Legacy Cookies
Internet Explorer 6 defines a legacy cookie as a cookie that exists on the user's computer at the time Internet Explorer 6 is installed or that is imported from another browser using the Import/Export Wizard under the File menu of Internet Explorer. Legacy cookies are not deleted during installation.

Legacy cookies are leashed when the default privacy setting is set to High, Medium High, Medium, or Low. Under the Block All Cookies or Accept All Cookies settings, legacy cookies are treated the same as other cookies and unconditionally blocked or accepted respectively.

Unless the user's privacy setting is set to Accept All Cookies, legacy cookies are accessible only in a first-party context.

6.1 Legacy Cookies Provisions
Internet Explorer 6 has made a special provision for opt-out legacy cookies. Often when a user opts out of some online service, an opt-out cookie is used by the Web service to facilitate this choice. The success of this transaction sometimes relies on the ability of the browser to send this cookie in both first- and third-party contexts. However, with Internet Explorer 6, legacy cookies are leashed and are not sent in the third-party context. In response to this problem, Internet Explorer 6 does not leash cookies where the name/value pair is "ID=OPT_OUT". (Please note that this name/value string is case- and space-sensitive.) Web sites should upgrade their opt-out cookies to use this syntax to ensure legacy opt-out cookies are effective in Internet Explorer 6. Once Internet Explorer 6 is installed, new opt-out cookies are handled just like any other non-legacy cookie.