tissue=number of that tissue which currently tolerates the lowest ambient pressure without symptoms — optional
The element indicates the actual value of the so-called "leading tissue" as a percentage as a real number. At a certain timestamp there exists only one "leading tissue". Therefore <gradientfactor> may appear only once inside a <waypoint>.
The attribute tissue indicates the number of the tissue whose value is given by the <gradientfactor> element.
(see also <waypoint>)
<!-- tissue no. 4 is that one with the highest value of --> <!-- 80 % of the maximum tolerated saturation --> <gradientfactor tissue="4">0.8</gradientfactor>