How to read datasheets for TTL logic gates regarding maximum output current conditions
The convention is that when the gate output is sourcing current it is given a negative sign and when it is sinking it is given a positive sign.

True TTL gates (such as 7400) actually did require current to bring the inputs high as the reverse Hfe of the multi-emitter transistor is not zero.  This current is typically shown as 40uA.

Later devices such as LSTTL typically use multiple diodes at the input and require much less current.