T-400 is a perfect blend of 3 testosterone compounds.
The 3 different testosterone esters provide for different half lives. Esterization of the testosterone molecules provides for a sustained (but non-linear) release of testosterone from the injection depot into the blood plasma.
While the intention of the mixed testosterone esters in test 400 is to provide more stable serum testosterone levels, the long-ester testosterones, such as testosterone cypionate or testosterone Enanthate, provide more stable serum testosterone levels.
Testosterone 400 is the most popular testosterone blend product among those that use anabolic steroids for muscle building purposes.