A transactional computer system comprises a plurality of entities including at least one entity of each of the following forms, a Thing entity having the properties of identifying a client system and uniquely identifying an object in that client system, a Proposal entity for defining a transaction, the Proposal entity being subordinate directly or indirectly to a Thing entity and having the properties of modelling at least one external agent to carry out a transformation in relation to the first entity, and a Decision entity capable of communicating with a Proposal entity and having the properties of defining the types of decision that may be made, and determining the responses in relation to those decisions. The system preferably further comprises at least one Assignment entity subordinate to an associated Thing entity, the Assignment entity having the properties of uniquely identifying the associated Thing entity, and identifying a particular type of assignment or transformation to be applied to the Thing entity. This entity may be combined with the Proposal entity. Additionally the computer system preferably comprises at least one Tender entity associated with a plurality of Proposal entities and a single Thing entity, and identifying at least a quantity.