ULPA stands for Ultra Low Particulate Air. An ULPA filter by definition removes 99.999% of particles .00012 of a millimeter in size. Serious stuff! ULPA filters have an even higher rating and filtration effect than HEPA filters and are some of the highest grade filters around for removal of ultrafine particulate matter from the air.
However, because the ULPA material is more dense than that of a HEPA Filter , airflow is more restricted - meaning less air circulation. One interesting thing to note is that ULPA units do not necessarily perform as well overall as HEPA units in providing particle-free air as they may not "change the air" in a room as frequently as a HEPA unit - due to their restricted airflow.
A better ULPA unit would compensate for the lower airflow by prodiving a larger surface area of filter media. It is also critical for ULPA air filters to be well-sealed as leaks can compromise the draw of air through the filter media.
ULPA filters may offer improvements in smoke filtration over HEPA filters; however well-made HEPA filters should theoretically provide adequate filtration to remove dusts, pollen, bacteria and insecticide dusts. It is more common to see HEPA filters used in combination with other technologes such as gas phase purification (including activated carbon) and UV light sterilization to address other airborne contaminants.
The ultimate ULPA filters are known as "SULPA" filters - Super ULPA. These have a higher efficiency rating.
It is worth noting that particle filtration through any given HEPA-type media is in some ways theoretical. The velocity of air through the medium also affects the efficiency - and this makes sense when one imagines that particles that may be caught at low air velocity might be forced through by higher air velocity. Also, filters with high efficiency will become loaded with dust more quickly - and performance characteristics vary during the life of the filter medium.
In short, filter efficiency has many complex variables and ultimately, real-world testing of the finished unit is the best way to measure actual results.