What ISML tag should be used to avoid code repetition for a common header and footer layout on multiple pages?

The most appropriate ISML tag to avoid code repetition for a common header and footer layout on multiple pages in Salesforce Commerce Cloud is to use the <isdecorate> tag. This tag facilitates the creation of a layout that can be reused across various templates. By implementing a decorator, you can define a base layout that contains the common elements such as headers and footers, allowing other pages to inherit this structure without needing to duplicate code.

The <isdecorate> tag wraps the content of a page and applies a defined template layout. This ensures that all pages maintain a consistent look and feel while promoting code reuse and making the maintenance of the codebase more manageable.

Other options, while useful in their own capacities, do not specifically address the scenario of creating shared layouts. For instance, <isreplace> is designed to replace the content of template variables but does not facilitate layout sharing. Similarly, <iscontent> is used to provide content placeholders, and <isinclude> allows for including partial templates but is not as suited for creating a whole layout structure as <isdecorate>. Thus, utilizing <isdecorate> proves to be the most efficient way to implement shared header and footer structures across multiple pages.