History

Grover Furr collects resources on his espresso Stalinist blog

https://espressostalinist.com/the-real-stalin-series/party-purges/

There are a ton there but I’ll highlight several

The general criteria for the purging of party members were corruption, passivity, breaches of party discipline, alcoholism, criminality and anti-Semitism. For bourgeois individuals and kulaks who hid their class origin expulsion was certain. (But not for those who had been accepted into the party and who had admitted their class background.) For the former tsarist officers who hid their past were also inevitably expelled. All those who had been expelled could in their turn appeal to the Central control commission, and then their cases were reviewed at a higher level. Sousa, Mario. The Class Struggle During the Thirties in the Soviet Union, 2001.

It would be a mistake to regard the 1933 chistka as having been directed solely against members of the opposition. The largest single group expelled were “passive” party members: those carried on the roles but not participating in party work. Next came violators of party discipline, bureaucrats, corrupt officials, and those who had hidden past crimes. Members of dissident groups did not even figure in the final tallies. Stalin himself characterized the purge has a measure against bureaucratism, red tape, removed, and careerists, “to raise the level of organizational leadership.” The vast majority of those expelled were fresh recruits who had entered the party since 1929, rather than Old Bolshevik oppositionists. Nevertheless, the 1933 purge expelled about 18 percent of the party’s members and must be seen as a hard-line policy or signal from Moscow. Getty & Naumov, The Road to Terror. New Haven, Conn.: Yale Univ. Press, c1999, p. 127

The arrests affected chiefly the upper party circles and those officials dealing with foreigners; hence they seemed to foreigners more extensive than they were. None of the arrests was as wanton as the foreign press portrayed them; evidence of some sort was indicated. The common sentence was not execution, but swift removal to another job in another part of the country. Fairly large numbers of such transfers seemed to have occurred merely on suspicion, on the theory that if suspects were guilty, or had guilty connections, the transfer would break these up; if they were innocent they would not suffer much from a job transfer and would come back to Moscow eventually if they chose. Naturally such people did not hasten to communicate with their foreign acquaintances during their absence, and this often led the latter to assume that the Russians had been “liquidated.” A year or two later, large numbers of such people returned, none the worst for their temporary job in the “sticks.” Strong, Anna L. The Soviets Expected It. New York, New York: The Dial press, 1941, p. 137

Between November 1936 and March 1939, including 1937, when the ‘Great Purge’ was at its most intense, roughly 160,000 to 180,000 people left the CPSU (for any reason). This represented about 8% of total Party members, far fewer than those who were expelled in the purge of 1933. In 1937, at the height of the Great Purge in Moscow, 33,000 (13.4% of the total Moscow Oblast Party organization) left to the party; this compares with 133,000 in 1933 and 45,500 in 1935. Szymanski, Albert. Human Rights in the Soviet Union. London: Zed Books, 1984, p. 241

Also you can read specifically about the military purges prior to WW2

https://espressostalinist.com/the-real-stalin-series/military-purges/