What is a True HEPA and how to select the correct filter

What is the difference between a Filter, HEPA Filter and True HEPA Filter?

HEPA or  "High Efficiency Particulate Air" typically it is a standard that means it has a filter that remove "at least 99.97% of particles from the air down to at least 0.3 microns".

The term true HEPA is a legacy word that originated in the 1940's as a standard created is the USA by the Department Of Energy (DOE) that conformed to a very high set of standards used ins areas that worked within radiation. There is  no governmental department that rates for "True HEPA" so instead of this term you should look for how they comply with a certain standard. Two common standards are set out below:

USA Standard - American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE)

European Standard - European - EN1822

Conclusion

The simplest conclusion is, to go with a trusted brand that claims to remove 99.97% of particles at a micron size of 0.1mm. 0.1 microns is important as this is the size of dust mite faeces which cause your allergies to trigger. 

I would recommend the Dyson V15 Detect Absolute cordless vacuum