<td class="selected"></td> now when it is used??

Question

html table tags

Answer 1

What do you mean? By setting class on an element you can style that element with CSS
.selected {
border 1px solid black;
}
or use querySelector to target this element with JavaScript and do something with it.
tdElem = querySelector('.selected')
tdElem.innerHTML = "Hello!"

Answer 2

that's similar to table data name