tab is a placeholder element for a singular horizontal tab character (HTML entity 	). This is needed because a tab character is whitespace that is often discarded by default by XSLT processors. Tabs can be used to separate items into lists to save space on a page (such as when items are short in length). You should consider whether using tabs is appropriate or if a table (or simple table) is a better option.
The tab character (HTML entity &09;) is difficult to represent in some outputs, especially HTML-based outputs like epub. One possible solution is to use five non-breaking spaces (HTML entity  ); the non-breaking space is a good choice because multiple regular spaces will be ignored by most HTML rendering. The downside to using non-breaking spaces is that you are not guaranteed your items will line up. If you want your items to line up neatly, then the best solution is to use a table.
<p>This is text before a tab<tab />This is text after a tab</p>