In what sense is language arbitrary

The U.S. linguists Bernard Bloch and George L. Trager formulated the following definition: “A language is a system of arbitrary vocal symbols by means of which a social group cooperates.” 
With respect to linguistics, refers to the fact that the sounds of words typically have no relationship to their meanings. 
There is no relationship between the sounds associated with the word and the physical object or action that the word signifies. For example, there is no natural reason why a MAN should be associated with / mæn / in English or [mesa] in Spanish. 
In terms of grammar, there are many different strategies possible for marking the relationship between the different nouns of a sentence. The order in which we place words within a sentence is also arbitrary and it is not based on any natural reasoning, but depends on how languages have naturally evolved. 
For example, there is no reason for saying 
in English - a big table 
in Portuguese - uma mesa grande