To sum this article up, it states the banks allow 75% of current rental income to be counted as an individuals income towards the new/future mortgage, and 75% of the potential future rental income on the property which the new/future mortgage is for to count as well (as long as you have 2+ years landlord experience).
Example: Your day job income is $80,000/year. You currently own a triplex that generates $1,000 each unit while 'house hacking' and living in the third unit. $1,500 of the $2,000 in rent counts as 'income', meaning you tack on $18,000 a year to the $80,000 day job income for a total income of $98,000 a year. Now you're at a bank trying to finance a mortgage on a quadplex that generates $1,000 rent on each unit. Since the bank will allow 75% of the potential rent to count towards income, you can now add $3,000/month of $36,000/year to the now $98,000 yearly income to make it $134,000 yearly income which the bank will finance you for; instead of just based off the $80,000 day job income.