Sequence data contains sequences of residues without structural data. The derivation and accuracy of such files is the subject of research but the formats tend to be simple.
Sequence information generated by the FASTA sequence alignment package.
The format of FASTA is described here
In this example 2ptl_ is the identifier (using the name of the PDB). 1-120 describes the range of residues the file contains. : Example B.9. FASTA