The words abhi and prati when implying direction (towards), are optionally compounded with a word ending in a case-affix denoting the limit, or the goal which regulates or marks the direction and the resulting compound is avyayībhāva,
The word lakshana means a sign goal. See also (1. 4. 84) and go for a further explanation of this word. As अभ्यग्नि or अग्निमभिलभाः पतन्ति 'the moths fall in the direction of fire.' So also प्रत्यभि or अग्निं प्रति, the sense being, that making the fire their aims (लक्ष्य) they fall towards it.
Why do we say 'when meaning lakshana'? Observe स्रुग्घ्नं प्रति गतः 'he turned back and went towards Srughna i.e., having torned back he went only towards the direction of Srughna but not to that place itself.
Why do we say the words abhi and prati? Observe येनाग्निस्तेन गतः 'he went in the direction of the fire' .
Why do we say 'when meaning direction towards'? Observe अभ्यङ्का गावः or प्रत्यङ्कागावः 'the recently marked cows.',