Must - a specific requirement in legislation or of a principle, rule, outcome or other mandatory provision in the SRA Handbook. You must comply, unless there are specific exemptions or defences provided for in relevant legislation or the SRA Handbook.
Should - good practice for most situations in the Law Society's view. If you do not follow this, you must be able to justify to oversight bodies why this is appropriate, either for your practice, or in the particular retainer.
May - a non-exhaustive list of options for meeting your obligations. Which option you choose is determined by the risk profile of the individual practice, client or retainer. You must be able to justify why this was an appropriate option to oversight bodies.
SRA Code - SRA Code of Conduct 2011
SRA - Solicitors Regulation Authority
The Law Society also provides a full glossary of other terms used throughout this practice note