The fixed setting is true for fonts where each character fits into the same-sized bounding box. The linespace is the distance between the baselines of successive lines. The ascent and descent are illustrated in Example 39-2:
Example 39-2 Font metrics.
The font measure command returns the width of a string that will be displayed in a given font. The width does not account for heavily slanted letters that overhang their bounding box, nor does it do anything special with tabs or newlines in the string.