The for attribute is an allowed attribute for <label> and <output>. When used on a <label> element it indicates the form element that this label describes. When used on an <output> element it allows for an explicit relationship between the elements that represent values which are used in the output.

When used as an attribute of <label>, the for attribute has a value which is the id of the form element it relates to.

<label for="username">Your name</label> <input type="text" id="username" />

When used as an attribute of <output>, the for attribute has a value which is a space separated list of the id values of the elements which are used to create the output.

<input type="range" id="b" name="b" value="50" /> +
<input type="number" id="a" name="a" value="10" /> =
<output name="result" for="a b">60</output>

See examples of usage on the element pages for <label> and <output>.