In Profile searches you can chose the number of allowed errors in comparison to profile ET (error threshold). This will be the number of "negative" patterns that may be present for the particular profile still to be matched. For the data used for training up to ET errors are safe (i.e. does not decrease the reliability of predictions), this may not be true for other submissions. Allowing a certain number of erros may be useful, if you submit a multi-domain structure or a domain that is untypical for CATH.