ClientId is about a the id property in the client applications such as applications written in html. Some technologies use the id property executed in the client-side, like html, javascript, cssand libraries based on these techonologies.
Javascript:
document.getElementById('productName').innerHTML = 'Some Text';
Css:
#productName {
color: #f00;
font-weight: bold;
}
Html:
<span id="ProductName"></span>
