Annex X-2 contains a number of technical definitions and acronyms applying within the BSC. The information is divided between those terms used only within Section S relating to Supplier Volume Allocation, and terms used throughout the remaining sections of the BSC. This is required because, for convenience, some definitions and acronyms have a different meaning between Section S and other sections of the BSC.

In addition, Annex X-2 also includes some interpretation rules relating to mathematical operations and timing conventions, and explanations regarding the use of subscripts, other expressions and summations both in terms of those that apply to Section S and otherwise.

The conventions used in relation to variables that are functions of time are described in Annex X-2. For example, reference is made to spot times and it is noted that a number of data items (e.g. Acceptance Data and Bid-Offer Data) contain values that are specified for a particular instant in time. These instants in time are referred to as spot times which, because they are instantaneous, have a zero duration.

Thus, the spot time at the end of Settlement Period j is the same time as the spot time at the start of Settlement Period j+1. Settlement Periods have a 30-minute duration commencing at the spot time at the start of the Settlement Day (00:00).

Further rules are contained in Annex X-2 relating to point variables, that is, where more than two values of a particular function are specified at any spot time (i.e. there is a step-change in the value), point value identification numbers are used. This allows steps up and steps down to be specified in the data. If there are two point values at the same instant in time, the one with lower point value identification number is processed first.

Rules relating to linear interpolation are also described. Linear interpolation is used to define continuous functions of time for all spot times in a certain period. Therefore some functions are defined across all spot times in a certain time interval. These functions are normally written as F(t). These functions are never actually evaluated, because they have an infinite number of values in any finite time interval. They are, however, used to define other parameters that are evaluated, typically by integrating the continuous function of time over a Settlement Period. A common misconception is that in order to evaluate the integrand in such cases, the value of the relevant function of time F(t) is evaluated for each minute of the Settlement Period, and that the average of these values is taken. This is not the case, and instead a true integration over all times is required in the BSC. It is possible to evaluate such integrals exactly because all functions requiring integration in the BSC are piecewise linear functions.

A number of Tables are also included in Annex-X-2 as follows:

Table X-1 – This sets down the subscripts and superscripts used, except in relation to Section S.

Table X-2 – This sets down a series of definitions of technical terms used in the BSC (other than Section S). In most cases, a "layman’s" description of the definition is also included in italics.

Table X-3 – This lists the acronyms used except in relation to Section S.

Table X-4 – This sets down the subscripts and superscripts used in Section S.

Table X-5 – This sets down the convention for use of summations in Section S.

Table X-6 – This sets down the definitions used in Section S.

Table X-7 – This lists the acronyms used in Section S.

Table X-8 – This lists the valid Consumption Component Classes (which may be amended by the Panel).

Table X-9 – This lists the Supplier Volume Reporting Groups used in the Supplier Quarterly Volume Report determined according to paragraph 9A of Section S, Annex S-2.

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