Dig pin 18 and 19

In hifive1.h only PIN_18_OFFSET & PIN_19_OFFSET values are given.
How SCl SDA digital pins 18 and 19 differs from plain digital pins 18 and 19?
Currently i am working with soft i2c so i have assigned PIN_18_OFFSET & PIN_19_OFFSET for SCL and SDA pins.In future if i want to use plain digital pin 18 and 19 along with scl & sda what should i do?
But i guess those offset values are working for both,how it is possible then, to access different devices communication using same PIN_X_OFFSET values?