What is a "data-ad-slot" for Google Adsense and why is inline styling required?

Question

I have this adsense ad code:

<script async src="http://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
<ins class="adsbygoogle"
    style="display:inline-block;width:728px;height:90x"
    data-ad-client="ca-pub-xxxxxxxxxxxxxxxx"
    data-ad-slot="6440411535"></ins>
<script>

source

What is a data-ad-slot for Google Adsense and why is inline styling required — aren't all ads have a fixed mobile-first styling?

_{This question is very much a follow up to this question.}

Answer 1

When you create an Adsense ad, you can select it to be either responsive or fixed size.

If it's a fixed size, you'll get the sizing thrown into the ad as part of the styling:

style="display:inline-block;width:728px;height:90x"

However, if you choose a responsive size, it will display something like this:

 style="display:block"
 data-ad-format="auto"
 data-full-width-responsive="true"

If you don't want to use inline styling, you can skip the style tag all together and define it on your css, for example:

<ins class="adsbygoogle myadstyle"

And in your css add something like:

.myadstyle{display:inline-block;width:728px;height:90x}

Regarding your second question, data-ad-slot is the code for the ad unit you've created. If you go to Ads > Ad Units, you'll see all the ads you've created for your account. Each of those has a unique ID which is the value used for that field.