You must not specify category in the a element. The value would be a URI ( category1.php ), not a content ( My category 1 ).
category expects a value that is either text or another element.
So, if you want to provide text, you can use something like:
<div itemscope itemtype="http://schema.org/Offer"> <a href="category1.php"><span itemprop="category">My category 1</span></a> </div>
And yes, it’s quite normal to use only one property. Schema.org does not define the required properties . But consumers (for example, search engines) can, of course, only consider the reuse of your data if it meets certain requirements.
UPDATE: if you want to provide several categories for an element, it could be something like this:
<div itemscope itemtype="http://schema.org/Offer"> <ul> <li itemprop="category"><a href="category1.php">My category 1</a></li> <li itemprop="category"><a href="category2.php">My category 2</a></li> <li itemprop="category"><a href="category3.php">My category 3</a></li> </ul> </div>
Do not rename the category property! Addendum 1 , 2 , etc. It would be wrong.
(And this is a hierarchy of categories, you can use a single value and / or > ., For example Sports > Tennis .)