"What is the meaning of ICUMSA and to what does it relate?" In order to attempt to help, let us quote the following from the ICUMSA Handbook:  

1.  "ICUMSA (International Commission for Uniform Methods of Sugar Analysis) is a world-wide body which brings together the activities of the National Committees for Sugar Analysis in more than thirty member countries. Work is carried out under various Subject headings, each headed by a Referee.  
Methods are recommended for tentative approval by ICUMSA in the first instance. Upon meeting all of the Commission's requirements, methods are accorded official status. Methods which are demonstrably useful and have found an established application, or which do not lend themselves to collaborative testing are given an Accepted status"  

2.  An ICUMSA rating is an international unit for expressing the purity of the sugar in solution, and is directly related to the colour of the sugar. Be aware that there are different types of ICUMSA units. 
For Brazilian sugar, the lower the ICUMSA figure the whiter the sugar. However, this is not the case in the E.U. for some unknown reason, which has been the subject of much discussion. 
SGS of Sao Paulo has published specifications for ICUMSA numbers for E.U. product which run contrary to the Brazilian specifications; for example, in Brazil SGS has an ICUMSA rating of 45 rbu for refined, indicating the highest quality, with other grades of lower quality (such as Special Extra Crystal) having a higher ICUMSA of 150 and so forth. This rating method is confirmed by the Institute of Sugar and Alcohol in Brazil.  

However, SGS of Sao Paulo, in the same memo, lists ICUMSA 46 for the highest EU grade and ICUMSA 42 for E.U. raw, which is opposite to Brazil. SGS itself in either Brazil or Switzerland is not much help in this area, although they have published the memorandum listing specifications and are supposed to be the authority.  

When one speaks to ICUMSA directly, they refer to a third method which is totally different. How does one know what one is buying?  

The only sure method is to be first totally versed in sugar as a starting point and understanding these differences. Next, although many brokers use the term 'rbu' with reference to ICUMSA for Brazilian sugar, few understand its meaning. RBU, as used in this sense, means "Reference Base Units". 

But to what it is referenced is the key. All major buyers such as Coca Cola etc have their own reference for this purpose. If you do not have a reference, or are not acceptable to the reference supplied by the refinery or SGS, then you would request sugar ICUMSA to be expressed in TU or ICUMSA Units. 

The TU is heavily dependent upon "ash points", which are defined as "Conductivity ash, %, expressed to three decimal places" (British Sugar Corp., Central Laboratory, ICUMSA Headquarters). Further used to weight the TU is "reflectance Grade Colour" and "Solution Colour" or "Filtered Colour" as it is referred to, which have formulas for determination equally confusing as the one for "Ash Points".  

The buyer is best protected by having an understanding of the method used by the provider to obtain the proper product complying with the proper specification. Country of Origin specifications usually rule. 

These specifications are then detailed in the contract.  

Traders should note that the term "INCUMSA" with an "N" is not a valid sugar trade term. (Also, it is interesting to see offers which include the words "Grade A", which of course no sugar refinery or seller uses!)