What makes a DNA sequence motif a DNA sequence motif?
Motif means a pattern. For any array of objects to be called a pattern, it has to have at least more than one instance. 
To define more precisely, a motif should have significantly (statistical) higher occurrence in a given array compared to what you would obtain in a randomized array of same components (expecting at random). 

[Here i use the term array to mean an arranged set such as a genome (array of nucleotides with their position in the sequence) or a protein structure (array of amino acids with their vector positions). A randomized array with same components is generated by shuffling the elements and their values; for e.g. assign the same collection of nucleotides to random positions] 

RSS nonamers/heptamers are called a motif because they are a pattern of sequence which a) has more than one occurrence b) the occurrence is significantly higher than a random expectation. Generally in sequence motifs randomization is not usually done because the genome size is too huge. In such conditions if your sequence has >= 2 occurrences throughout the genome it may be considered a motif. Lets assume a 10bp sequence. Its random probability = 1/4¹⁰. Therefore in 1048576 bp (1Gb) there is a likelihood of finding one instance of that sequence on random. So I assumed that if that sequence has >=2 instances it can be called a motif. However, I am not sure about this.

RSS is not a motif because the entire RSS is not conserved; the conserved 'motifs' are interrupted by sequences. This is analogous to lets say a promoter which is a functional region of the DNA. Promoter can have binding sites for different transcription factors and these operator sites (TFBS) can constitute a motif (if they are conserved) but not the entire promoter.