Historically, accounting systems worked on the basis of a single string code, which contained all the information needed.

e.g. 10011231122 where 1001 might be revenue, 123 might relate to cost/profit centre and 1122 might relate to country 

You would then require multiple accounts based on each of these elements.

i.e. 10011241122 for revenue in a different cost/profit centre

or 10011231121 for revenue in a different country

etc.

Code Parts effectively split this single string code out into different parts corresponding to each element:

i.e. Account 1001 / Cost Centre 123 / Country 1122

A code string then pieces these elements together to recreate the single string code seen in historic systems.

Code Parts and Strings make your chart of accounts much more flexible and much easier to manage, as adding a new cost centre only require creation of a single code part value rather than multiple single string accounts.

Hope this helps :)

Code parts are elements of the general ledger.   It allows the client to define segments for reporting and analysis.   Posting controls map the transactions based on various basic data points.    An example would be the ability to define a revenue account (code part A) and then one or more subsequent elements (code parts) such as market, region, business unit.