where to use the
post_class()
You can use it pretty much everywhere (in WordPress), e.g. single post pages (single.php, single-my_cpt.php, etc.), search results pages (search.php), the index template (index.php), a template part (e.g. content.php), and a shortcode function.
it is ok to use
post_class()for adding custom classes?
Yes, it is. post_class() is not restricted to WordPress default classes only.
So for example, post_class( 'my-class' ) will work unless if a code filters the classes and then removes the my-class from the list.
it applied to all other files like the about page, contact page
Restricting it (your custom CSS class) to be applied only to certain pages is your task and WordPress won’t automatically apply the restriction for you.
But WordPress provides various conditional tags like is_page() that you can use to add a CSS class only on certain pages, hence just use the relevant conditional tag with post_class().
For example, to add my-class only on a Page (post of the page type) with the slug about:
<div <?php post_class( is_page( 'about' ) ? 'my-class' : '' ); ?>>
your code
</div>
<!-- or maybe like so: -->
<?php
// foo-bar would always be in the list
$classes = [ 'foo-bar' ];
if ( is_page( 'about' ) ) {
$classes[] = 'my-class';
}
?>
<div <?php post_class( $classes ); ?>>
your code
</div>
And note that post_class() by default uses the current post in the current WordPress query, i.e. the post referenced to by the global $post variable, so if you want to output the classes for a specific post, pass the post ID (or object) to post_class():
// 123 (2nd parameter for post_class()) is the post ID
<div <?php post_class( 'my-custom-class', 123 ); ?>>
your code
</div>