From the SWJ Reference Library-Afghanistan website

The Sarandoy served as a national police force, but their armaments surpassed the traditional police arms of pistol, baton and shotgun. The Sarandoy constituted a third ground force within the DRA. They had heavy armaments, armored personnel carriers and a separate command and control system. The DRA Army, KHAD and Sarandoy often worked together out of necessity, but they were separate, rival systems designed to counterbalance one another and prevent regime ouster. It was not an efficient, or particularly effective design, but the DRA was designed for regime survival, not efficiency or effectiveness. Furthermore, the DRA leadership saw their chief threat as internal subversion within the communist party instead of the rural Mujahideen.
From the Illinois Institute of Technology website

The general ineffectiveness and unreliability of the Afghan army led the Kabul regime to organize a number of paramilitary internal security forces. Probably the most important of these in the mid-1980s was the Sarandoy (Defenders of the Revolution), an armed body under the control of the Khalqidominated Ministry of Interior. It was an outgrowth of the Daoud-era Gendarmerie that before 1978 had comprised about 20,000 men. The November 1985 issue of Jane's Defence Weekly gave approximately the same figure for the size of the Sarandoy in 1985. It was organized into six brigades or regiments, numbering around 6,000 men and based in Qandahar, Badakhshan, Baghlan, and Parvan provinces and in Kabul, which had two Sarandoy units; there were also 20 operational and mountain battalions, with an additional 6,000 men; personnel attached to the national and 28 provincial headquarters of the Sarandoy, numbering around 3,000 men; and other personnel attached to the Sarandoy Academy and to administrative, construction, and maintenance units. These totaled a further 4,000 men. Established in early 1981, the force played an active role in offensives against the mujahidiin, though its effectiveness was hampered by the rivalry between Parchamis and Khalqis. Sarandoy relations with Parcham-dominated KHAD were tense.
From wikipedia

There was a bitter rivalry between Najibullah and Sayed Muhammad Gulabzoi. Gulabzoi, a Khalq sympathizer, was Minister of the Interior and commander of Sarandoy ("Defenders of the Revolution"), the national gendarmerie. Gulabzoi was one of the few prominent Khalqis remaining in office in a Parcham-dominated regime.