Added information to #form

I like to know correct name within html if I add information like:
section#form

How to set correct name within html like:
<dd id="form">

as there is added section to define groups or even more ID like:
section#form dt#myforms

I like to know correct name within html if I add information like:
section#form

How to set correct name within html like:
<dd id="form">

as there is added section to define groups or even more ID like:
section#form dt#myforms

What?

I will be more clear. Let us start from 0.

If I add section# what to change at website HTMl code not CSS that section will be recognised?

Is this called as comment or group or this is important to put inside my website code?