Transcript of unknown function
Transcripts of unknown function (TUFs) is the name that has been suggested for known RNA transcripts of DNA whose function is unclear. Most are probably ncRNAs, such as RNAi or snoRNAs, but could also represent a whole new class of ncRNA. Their DNA sequences reside in the intergenic or intronic regions of the genome, which is often called junk DNA. Categories 
Broadly speaking, TUFs can be classified into three categories:
 TUFs that are complementary to sense transcripts of protein-coding genes
 TUFs that are novel isoform transcripts of protein-coding genes; this can include expressed pseudogenes
 TUFs that reside on the same strand as protein-coding genes in the intronic region or entirely in the intergenic region

References 

Molecular genetics
RNA
Non-coding RNA