Without enough current the circuit will not maintain a valid logic level. Read more from TI handbook on TTL below
OL Low-Level Output Current
JEDEC – The current into the output terminal with input conditions applied that, according to the product specification, will establish a low level at the output.
TI – The current into an output with input conditions applied that, according to the product specification, establishes a low level at the output.
TI data sheets specify currents flowing out of a device as a negative value. IOL maximum is used as a test condition for VOL. See VOL testing for details.Logic output drivers have a maximum current-drive capability that they can sink and still be able to sustain a valid logic-low level. In a static dc state where current is continuously drawn into the output, because CMOS drivers operate in the linear region, their behavior will be somewhat like a low-impedance resistor and will increase in voltage potential (i.e., increase the VOL level) as the increasing current is sunk into the output pin during a VOL test. Consequently, a TI logic device will operate with a low-level output current that is above the recommended operating range (but below the absolute maximum rating), but TI does NOT represent that the device can sustain the specified VOL level or that the device will operate without any reliability concerns.
IOLS Static Low-Level Output Current
JEDEC – no definition offered
TI – The static and testable current into a Dynamic Output Control (DOC circuitry) output with input conditions applied that, according to the product specifications, establishes a static low level at the output. The dynamic drive current is not specified for devices with DOC circuitry outputs because of its transient nature; however, it is similar to the dynamic drive current that is available from a high-drive (nondamping resistor) standard-output device.
TI data sheets specify currents flowing out of a device as a negative value.
DOC circuitry is designed to drive CMOS input devices, which are capacitive in nature, in point-to-point applications (one receiver input per driver output). For this reason, a large static low-level output current is not required. What matters most in this case is the high-transient-drive capability of the output.
For additional information about DOC circuitry, refer to the TI application report, Dynamic Output Control (DOC) Circuitry Technology and Applications, literature number SCEA009.