EtherCAT SII: What is 'receive mailbox' and 'send mailbox'?

The EtherCAT ETG 1000.6 V1.0.32 specification defines in section 5.4 SII coding that in the SII header, the following fields contain mailbox settings:

• Bootstrap Receive Mailbox Offset — 0x0014 (Unsigned16): Receive Mailbox Offset for Bootstrap state (master → slave)
• Bootstrap Receive Mailbox Size — 0x0015 (Unsigned16): Receive Mailbox Size for Bootstrap state (master → slave). Standard mailbox size and Bootstrap mailbox can differ — a larger Bootstrap mailbox may be used for optimization.
• Bootstrap Send Mailbox Offset — 0x0016 (Unsigned16): Send Mailbox Offset for Bootstrap state (slave → master)
• Bootstrap Send Mailbox Size — 0x0017 (Unsigned16): Send Mailbox Size for Bootstrap state (slave → master)

As the receive mailbox is used for master → slave communication, this is - as most other specifications in the EtherCAT standard - from the slave’s perspective.

Hence,

• The receive mailbox is MbxIn (Mailbox Input), for Master->Slave communication, and SyncManager SM1 shall be used for that
• The send mailbox is MbxOut (Mailbox Output), for Slave->Master communication, and SyncManager SM0 shall be used for that

Source: ETG 1000.6 V1.0.32, section 5.4 SII coding - requires free ETG membership.