44ab

Under section 44AB we have that if any person opt for presumptive taxation under section 44AD and if he offers below 8% and his income is above MAXIMUM AMOUNT NOT CHARGEABLE TO TAX, he is liable to tax audit under sec 44AB.so what is MAXIMUM AMOUNT NOT CHARGEABLE TO TAX here? does it mean BASIC EXEMPTION LIMIT? or If  exists tax liability  i.e after giving all deductions under sec 80C to 80U,if he is liable to pay tax ?if basic exemption is criteria,then automatically every firm offers less than 8% come under the ambit of sec 44AB as there is no Basic exemption limit for a firm and taxable at flat rate of 30%?