802.1AS-2011 allowed only one domain, i.e., domain 0, and required that the timescale be the PTP timescale. It specified in Table 10-6 (of 802.1AS-2011) that the common header flag ptpTimescale be reserved as TRUE and ignored on receipt. Note that 10.5.1 of 802.1AS-2011 specified that “Reserved fields shall be transmitted with all bits of the field 0 and ignored by the receiver, unless otherwise specified. (Note that the description in 802.1AS-2011 “Reserved as TRUE” is somewhat confusing, given the statement that reserved fields shall be transmitted as 0; however, the phrase “reserved as TRUE” is not used in 802.1AS-2020.)” 802.1AS-2020, allows multiple domains. It requires that the timescale be PTP on domain 0; however, the timescale can be PTP or ARB on other domains. In Table 10-9 (Values of flag bits), the value for ptpTimescale is described as “The value of the global variable ptpTimescale (see 10.3.9.7)”. Subclause 10.3.9.7 describes ptpTimescale as “A Boolean variable whose value is TRUE if the timescale of the current Grandmaster Clock is PTP (see 8.2.1) and FALSE if the timescale is ARB. This variable is used by both the BMCA and the explicit port state configuration option.” For backward compatibility with 802.1AS-2011, it needs to be indicated in the description of the ptpTimescale flag, in Table 10-9, that for domain 0 the flag is transmitted as TRUE and ignored on receipt.