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 - Outside of a regulatory context, good practice for most situations in the Law Society's view. In the case of the SRA Handbook, an indicative behaviour or other non-mandatory provision (such as may be set out in notes or guidance).
These may not be the only means of complying with legislative or regulatory requirements and there may be situations where the suggested route is not the best possible route to meet the needs of your client. However, if you do not follow the suggested route, you should be able to justify to oversight bodies why the alternative approach you have taken is appropriate, either for your practice, or in the particular retainer.
May - A non-exhaustive list of options for meeting your obligations or running your practice. Which option you choose is determined by the profile of the individual practice, client or retainer. You may be required to justify why this was an appropriate option to oversight bodies.
SRA Code - SRA Code of Conduct 2011
SRA - Solicitors Regulation Authority
IB - Indicative behaviour
The Law Society also provides a full glossary of other terms used throughout this practice note.