Basic question about the MRF24J40 RF chip?

Hello,

Can someone explain to me what MC means when they refer to symbols on page 98 (Figure 3-4 -Super frame structure-) of the following spec sheet:

https://download.mikroe.com/document...-datasheet.pdf


>>>> Backoff Period (aUnitBackoffPeriod = 20 symbols ?

What are symbols ?

Thank you for your help