An expression chain is a list of expressions separated by '|' characters.
While evaluating expressions separated by '|', PHPTAL will stop its evaluation when an expression value is not null and no error was raised while evaluating the expression.
As a string: expression is always true, string: always terminates an expression chain whatever expression may follow.
You can use php: expressions inside expression chains, like any other expression.
<h1 tal:content="page/title | page/alternativeTitle | php:get_default_title()" />