What does .container mean. 31 25 25 in CSS?

I’m doing a self-study on a responsive site. In a tutorial I got, there are in the CSS file instructions like:

.container. 25 {
    width: 100%;
    max-width: 1750px;
    min-width: 1400px;
}

.container. 5 {
    width: 1050px;
}

.container. 0 {
    width: 700px;
}

.container. 5 {
    width: 350px;
}

What is the meaning of:

.container. 25 
.container. 5
.container. 0
.container. 5

According to the CSS specification , they are escape characters, because the identifiers of the CSS can not have special characters, or better, they can only have characters aZ and 1-9, characters ISO 10646, hyphen and underscore.

.container. 25 = .container.125%
.container. 5  = .container.75%
.container. 0  = .container.50%
.container. 5  = .container.25%