lib/less/parser/parser.js
function () {
                    if (parserInput.peekNotNumeric()) {
                        return;
                    }

                    var value = parserInput.$re(/^([+-]?\d*\.?\d+)(%|[a-z_]+)?/i);
                    if (value) {
                        return new(tree.Dimension)(value[1], value[2]);
                    }
                }
function () {
                    if (parserInput.peekNotNumeric()) {
                        return;
                    }

                    var value = parserInput.$re(/^([+-]?\d*\.?\d+)(%|[a-z_]+)?/i);
                    if (value) {
                        return new(tree.Dimension)(value[1], value[2]);
                    }
                }
A Dimension, that is, a number and a unit  0.5em 95%