Any general text which is important for filling in the form and cannot be put into a hint, should be in that legend (as shown in the checkbox example). But the legend shouldn't be too long either.
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Fieldset
Use fieldsets to group related form controls. The first element inside a fieldset must be a legend element which describes the group.
Example
What bins do you currently have?
Select all that apply
HTML Snippet for fieldset
<h1 class="heading-medium">What bins do you currently have?</h1>
<p>Select all that apply</p>
<form>
<div class="form-group">
<fieldset>
<legend class="sr-only">What bins do you currently have?</legend>
<label class="block-label" for="waste-type-1" data-target="waste-type-1">
<input type="checkbox" id="waste-type-1" name="waste-types" value="Black bin (General waste)" aria-controls="waste-type-1" aria-expanded="false">
Black bin (General waste)
</label>
<label class="block-label" for="waste-type-2" data-target="waste-type-2">
<input type="checkbox" id="waste-type-2" name="waste-types" value="Brown bin (Garden waste)" aria-controls="waste-type-2" aria-expanded="false">
Brown bin (Garden waste)
</label>
<label class="block-label" for="waste-type-3" data-target="waste-type-3">
<input type="checkbox" id="waste-type-3" name="waste-types" value="Green bin (Recycling)" aria-controls="waste-type-3" aria-expanded="false">
Green bin (Recycling)
</label>
</fieldset>
</div>
</form>