F-tile-ethernet-hard-ip TX/RX MAC Segmented Client Interface

https://docs.altera.com/r/docs/683023/25.1.1/f-tile-ethernet-hard-ip-user-guide/tx-mac-segmented-client-interface 

"i_tx_mac_inframe" signal is explained as

" Indicates valid data in each segment for specific rate. Along with the previous segment's inframe signal, this signal indicates the SOP and EOP location."  i dont understand the underlined part of explanation. how does i_tx_mac_inframe indicates start of packer (SOP) and end of packet (EOP).

can someone please elaborate on this with a couple of examples 

also another question is

how to interpret i_tx_mac_data signals if i_tx_mac_valid == 1'b1 and i_tx_mac_inframe [15:0] == 16'h0 ?