What does this mean?
This is a note to check that where you are using a prefix or suffix on a text input, that you are referencing the contents of the prefix or suffix in the label.
For example with an input such as the one below we are relying on the presence of the suffix to tell the user what unit of measurement they should enter their data in:
<label for="weight">
Weight
</label>
<div>
<input id="weight" type="text">
<div aria-hidden="true">kg</div>
</div>
Impact on users
The impact is for screen-reader users.
Because the prefix/suffix is hidden from screen-readers (via the aria-hidden="true") this means they do not benefit from this additional clue about what they need to enter. This means they could enter the wrong data by mistake.
So we need to ensure the label is explicit about what type of data is being requested.
How to fix
When writing the label for the input, assume the prefix or suffix are not present and you are dealing with a plain text input. How would you ensure the user knew what they need to enter?
For example by adding "in kilograms" to the label we make it very clear for those who cannot perceive the suffix:
<label for="weight">
Weight, in kilograms
</label>
<div>
<input id="weight" type="text">
<div aria-hidden="true">kg</div>
</div>
It is also worth checking that your validation still allows whatever is in the prefix/suffix to be entered as part of the user-entered data.
References
GOV.UK guidance on prefixes and suffixes