The PDIF sign is basically the presence of a fibrosing ILD typically compacted within the last 2 cm of the peridiaphragmatic lung and may suggest not just CTD-ILD but specifically myositis ILD
When faced with indeterminate interstitial lesions, it is a good idea to ask for a COVID-19 history and if present, try and compare with those scans to understand whether the interstitial lesions are sequelae, new lesions or pre-existing prior to the COVID-19 episode