Rule 10B-18 covers the manner of repurchase, the time of the repurchase, the prices paid and the volume of shares repurchased. Compliance with the rule is voluntary. However, to fall within the safe harbor, the repurchaser must satisfy each of the four conditions daily. Otherwise, repurchases will not fall under the safe harbor for that day. https://www.investopedia.com/terms/r/rule10b18.asp The Four Conditions The manner of purchase: The issuer or affiliate must purchase all shares from a single broker or deal during a single day. Timing: An issuer with an average daily trading volume (ADTV) less than $1 million per day or a public float value below $150 million cannot trade within the last 30 minutes of trading. Companies with higher average trading volume or public float value can trade until the last 10 minutes. Price: The issuer must repurchase at a price that does not exceed the highest independent bid or the last transaction price quoted. Volume: The issuer cannot purchase over 25% of the average daily volume.