How to add the functionality of various attributes or tags to the search form ?

In the search availability form possible to use additional parameters for searching. For example searching by specific location or some other attributes.

How to configure it?

"Custom fields".
You can use the "custom fields" in the posts or pages, were you inserted shortcode for booking form.

One general rule for the "custom fields" - each custom field must start from "booking_" term.

For example, if you wan to search by City, then you need to make this configuration:

1) In the search form (at the Booking > Settings > Search page), you can use this selectbox configuration:

<label>City:</label>
<select id="booking_city" name="booking_city">
   <option value="">Any</option>
   <option value="Salernes">Salernes</option>
   <option value="Barjols">Barjols</option>
   <option value="Lorgues">Lorgues</option>
</select>

2) Then at the post or page, where you inserted booking form, you need to add new "custom field" with name "booking_city" and value (for example): "Lorgues".

In the same way you can add new additional parameter(s).
For example:

<label>Pool:</label>
<select id="booking_pool" name="booking_pool">
 <option value="">Any</option>
 <option value="1">Yes</option>
 <option value="0">No</option>
</select>

And of course in the page or post you need to create custom field: "booking_pool" with value "1" or "0"

General rules.

If you are having additional parameters, like this
- property_ab (this is some parameter)
- property_ac (number of beds)
- property_ad (square meters)
- ... etc...

you need to have parameters in format, where names of the custom fields starts from "booking_" terms:

- booking_property_ab (this is some parameter)
- booking_property_ac (number of beds)
- booking_property_ad (square meters)

Then inside of the search form you can have this selections:

    Number of beds:
    <select id="booking_property_ac" name="booking_property_ac">
    <option value="1">1</option>
    <option value="2">2</option>
    </select>

And for the posts or pages you can use custom fields with names "booking_property_XX", where XX = {ab | ac | ad}.

Tags
You can use "tags" in the posts or pages, where you are inserted the booking form for the specific booking resource.
In the search form you are need to use this keyword:

[search_tag] - search inside of posts/pages, which are have this tag.

Please note, its text field, so your visitors need manually to enter the tag for the searching.

Trick
Its also possible to customize this element as selectbox instead of the text field. You will be need to replace the [search_tag] shortcode to this HTML element:

<select id="booking_search_tag" name="tag">
<option value="">Any</option>
<option value="room">Room</option>
<option value="apartment">Apartment</option>
</select>

where "<option value="room">Room</option>" - list of options to select; "room" its value of the tag, and "Room" - its label, which is showing for the visitors....

Please click at the "Reset cache" button at the bottom of the Booking > Settings > Search page, after you are finish configuration, before making tests at the client side. You can check this troubleshooting instruction, if you do not see search results: http://wpbookingcalendar.com/faq/no-search-results/