Use classes for screen readers

Hi Anthony,

In the style.css there are 3 options to hide elements but keep them visible for screenreaders:

.screen-reader-text
.assistive text
.visuallyhidden

But I’m confused; How are they supposed to be used? This is my main question, but if it helps I split it up in separated questions:

1.
Why is class .screen-reader-text not used and / or replaced by class .assistive text?

2.
The class visuallyhidden in the style.css (taken over from html5boilerplate) looks almost identical with the code explained on www.ads-software.com, https://make.www.ads-software.com/accessibility/2015/02/09/hiding-text-for-screen-readers-with-wordpress-core/ .
a.
Why is the class .visuallyhidden used only once – in the header template – instead of assistive text, or the other way around?
b.
Why is the class .screen-reader-text not used?

Thanks in advance!
Sandra
Code snips below:
/* Text meant only for screen readers. */
.screen-reader-text {
clip: rect(1px, 1px, 1px, 1px);
position: absolute !important;
height: 1px;
width: 1px;
overflow: hidden;
}

/* Text meant only for screen readers */
.assistive-text {
clip: rect(1px 1px 1px 1px); /* IE6, IE7 */
clip: rect(1px, 1px, 1px, 1px);
position: absolute !important;
}

/* Hide only visually, but have it available for screenreaders: h5bp.com/v */
.visuallyhidden {
border: 0;
clip: rect(0 0 0 0);
height: px;
margin: -px;
overflow: hidden;
padding: 0;
position: absolute;
width: px; }