Liquid funds are those that are defined as money market funds in the offer document and invest in money market instruments of residual maturity up to 91 days. What sets liquid funds apart in terms of lowest volatility in returns among all categories of funds is that there is no mark-to-market (MTM) of the portfolio on a daily basis unless there is a trade in the secondary market in the underlying security (or securities ).
Practically there is no trade in money-market instruments and valuation of daily NAV happens on an accrual basis, i.e., by adding the coupon accrued for the day without any mark-to-market impact. Ultra short-term bond funds (USTBs) are defined as debt funds in the offer document and the fund manager is free to take securities of maturity longer than 91 days, but there is no compulsion to do the same.
Valuation of daily NAV happens as per the valuation matrix provided by the rating agencies, hence there could be a markto-market impact. However, the component of securities with residual maturity more than 91 days (for which the valuation matrix is published) is maintained on the lower side, so that the volatility in returns is limited. Returns in USTBs are marginally higher than liquids by virtue of the marginally higher portfolio maturity.
Normally, the longer the maturity of the instrument, the higher is the yield on that instrument (time value of money). USTBs have the liberty to purchase securities with maturity of more than 91 days and on an average, the portfolio maturity is longer than liquids funds. This leads to a higher accrual rate on the portfolio of USTBs. The volatility of returns in USTBs is marginally higher than liquids funds due to the small MTM component.
Operational aspects:
In liquid funds, purchase is T-1, i.e., cleared funds are given to the AMC by the cut-off time of 2 pm and availing of previous day's NAV. Redemption is T+1, i.e., the redemption request is placed within the cut-off time of 3 pm and the proceeds are received the next day.
In USTBs, purchase is on a T+0 basis, i.e., that day's NAV would be applicable. If the amount is more than Rs 1 crore, clear funds have to be given to the AMC by the cut-off time of 3 pm, otherwise the NAV of the day on which clear funds are being given will be applicable. Redemption is T+1 for USTBs as well. For both fund categories the application should be submitted and time stamped at the AMCs'/RTA's office within the cut-off time.
Source: http://economictimes.indiatimes.com/personal-finance/mutual-funds/analysis/the-nuances-between-liquid-and-ultra-short-term-bond-funds/articleshow/8023215.cms