Suspension Peroxide Oxidation Combined Acidity and Sulfur (SPOCAS) method is a suite of tests used to determine the net acidity via the measurement of: Titratable Peroxide Acidity (TPA) (as a measure of PSA); Titratable Actual Acidity (TAA); Excess Acid Neutralising Capacity (ANC); and Residual Acid Soluble Sulfur (S) or Net Acid-Soluble Sulfur (S) (as measures of retained acidity).   
As SPOCAS is effectively a self-contained ABA test, it is very useful and popular. However, the use of peroxide digestion for TPA may overestimate PSA where organic matter is present (see discussion on S). Therefore, it is recommended to measure S for at least one sample per batch of samples to determine if there is a substantial discrepancy between PSA measured via S and TPA. Additionally, note that S does not reliably measure the concentration of jarosite or schwertmannite (the typical sources of retained acidity); and S typically only accounts for 50-60% of jarosite. Consequently, S should be doubled to obtain a more accurate measure of Retained Acidity where jarosite is likely the principal source of Retained Acidity (which is the usual case). This calculation may or may not be performed by commercial labs before they present the ABA results to the client.   
Retained Acidity is a very recalcitrant pool of acidity in that it is slow to release and not easily neutralised. Therefore, soils with high amounts of Retained Acidity may require special management considerations to mitigate gradual acidification over tens to hundreds of years.