At the moment the platform lays out reach line of the address for the envelope and it includes a line called CRM Number, this is a placeholder for when a campaign has crm info relevant to that recipient for example on our Macmillan campaigns.
You do not need to write CRM Number on any envelopes for your job and this line can be ignored unless it is relevant to your campaign such as Macmillan in which case it should not be written on the envelope but rather the sys form according to campaign instructions.