Unknown @ rule in SCSS that creates-extends classNames?

Question

I am migrating an electron app into a web-browser app and I have come across some scss styling I have never seen before. Most of the selector use an At-Rule that does not make any sense.

It seems to generate class names but I am struggling to understand whether is a SASS features or some other library\package.

all these @b, @m don't make any sense to me.

I have googled but could not find better info neither on SCSS docs.

/* Text inputs */
@b textInput {
  @include inputBoxStyle;

  @m small, s {
    padding: 0.5rem;
    border-radius: 3px;
  }

  @m large, l {
    padding: 1rem;
    font-size: 1.2rem;
    border-width: 2px;
    border-radius: 5px;
  }

  @m textOnly {
    background-color: transparent;
    border: none;
    padding: 0 !important;
  }

  @m invalid, invalid:focus {
    border-color: $pastelPink;
  }

  @e hiddenInteraction:not(:hover):not(:focus):not(:active) {
    @include inputHiddenInteraction;
  }
}

/* Selects */
@b select {
  position: relative;
  select {
    @include inputBoxStyle;
    padding-right: 1.6rem;
    appearance: none;
  }

  &:after {
    position: absolute;
    top: calc(50% - (1em / 2));
    right: 1rem;
    font-family: 'FontAwesome', serif;
    font-size: 1rem;
    line-height: 1em;
    content: '\f078';
    color: $ourWhite;
    pointer-events: none;
  }

Answer 1

Thanks to the help of another developer I was able to figure out that this syntax is part of the package: https://www.npmjs.com/package/postcss-atrule-bem

Using the same SASS nesting adopted by the npm package, you can find and replace in your IDE the @b block with .block, @e element with &__element and @m modifier with &--modifier.