What is an open reading frame?
An open reading frame (ORF) is a stretch of sequence that starts with a start codon (ATG) and continues, codon by codon, until a stop codon. Long ORFs are candidate protein-coding genes, because random sequence contains a stop codon roughly every 21 codons.
Choosing a minimum length
Short ORFs occur by chance throughout any sequence. A minimum of 75 to 100 codons is a common starting point for bacterial genes; lower thresholds find small peptides but many more false positives. ORF finding is a first step, not a gene prediction: eukaryotic genes contain introns that split the coding sequence.
Reading the coordinates
Coordinates are 1-based positions on the forward strand of your input, including the stop codon. ORFs on the minus strand are reported with the same forward-strand coordinates so they can be compared directly.