Boeing sells aircrafts to airline companies.  As aircrafts are very expensive to build, they are only built "on demand", meaning that first a sales agreement is made with a customer, before the airplane is actually built. (An exception are demo versions of airplanes, but these are out of scope for this case). The sales are regulated by means of contracts with the airline companies, whereby a single contract may consist of several acquisitions of airplanes. The global contract stipulates common elements across all acquisitions such as delivery conditions, legal aspects, etc. Each acquisition of an airplane has further specific details, such as the chosen model of airplane, the negotiated price for that airplane, chosen options & customizations, delivery date, etc. Each contract is managed by a Boeing salesperson. An employee can act as salesperson for several contracts. Given the long term of contracts, the assigned salesperson may change over time, but Boeing ensures there is always a salesperson available for the client. 
Some airlines are related to each other: for example, main airlines often have a low cost daughter airline company. Boeing therefore keep track as much as possible of the mother-daughter relationships between airline companies, to be able to track whether to sold aircrafts are shifted to partner airlines of the original buyer.