तद् रूपकमभेदो य उपमानोपमेययोः। That is because of great similarity, there is no difference between उपमेय and उपमान i.e., उपमेय and उपमान are identical. When the intention is to show this, the figure of speech is metaphor.
सोत्तीर्णा खलु पाण्डवैः रणनदी कैवर्तकः केशवः।
(Pandavas swam across the river of war because Keshava - Krishna was their charioteer)
Here war and river are two objects - out of these two war has been compared with river. War is (रण) उपमेप, river (नदी) is उपमान with the assumption on that there is a great similarity between उपमेय and उपमान ie., war and river, here both are presented as one, so it is metaphor.
(Note: Unlike similie, there is not a comparison, it is assumed that उपमेय and उपमान are identical. However, upmeya and upamana are different, the difference between the two is also evident yet, by assuming a great similarityu between the two, the speaker mentions them as identical. For example, here upameya रण (war) and upamana नदी (river) both are different things. The difference between the two is very evident yet here having shown a great similarity between the two, it is metaphor in रणनदी.)