Marketable securities- Those securities which can be liquidated over a short period of time.
Non marketable securities – These include savings in fixed deposits having a certain locking period.
These securities are tax free, secure against dollar appreciation and is a reason for large foreign investments. These were issued at fixed rates by the US treasury department but were later shifted to an incremental rates to keep a check on the government debt levels by rate fixing . However recently auctioning of treasury securities has also taken place.