the regulations. Instead, it points to source documents by identifying their name and indicating the relevant paragraphs and clauses. This connection is created using "concept-reference" arcrole. There are several types of references that could be provided for each element. <reference xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/presentationRef" xlink:label="CashFlowsFromUsedInOperationsTotal_ref"> <ref:Name>IAS</ref:Name> <ref:Number>7</ref:Number> <ref:Paragraph>14</ref:Paragraph> </reference> <reference xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/measurementRef" xlink:label="CashFlowsFromUsedInOperationsTotal_ref"> <ref:Name>IAS</ref:Name> <ref:Number>7</ref:Number> <ref:Paragraph>18</ref:Paragraph> <ref:Subparagraph>a</ref:Subparagraph> </reference> The example above indicates references for Cash Flow from (Used in) Operations. First, it provides a reference to a document which explains how and where the element should be presented in terms of its placement and labeling. In IAS 7, paragraph 14 we read that the concept Cash Flows from Operating Activities exists and what it is derived from. Second, the measurement reference provides explanations about what determines the value of the element and how it should be calculated. This description can be found in IAS 7 paragraph 18.a. XBRL also allows an element to be assigned other types of references containing examples, commentaries, etc. Calculation Linkbase This linkbase associates concepts with other concepts so that values appearing in an instance document may be checked for consistency. The idea of the calculation linkbase is to improve the quality of an XBRL report. It contains definitions of basic validation rules, which apply to all instance documents referring to a particular taxonomy. A hierarchical calculation linkbase sorts all monetary elements in this way so that lower level elements sum up to or are subtracted from one another so that the upper level concept is the result of these operations. The sign of the relationship depends on the weight attribute that is assigned to the arc connecting two elements. An example is provided below. <calculationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="GrossProfit" xlink:to="RevenueTotal" order="1" weight="1" use="optional"/> <calculationArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/summation-item" xlink:from="GrossProfit" xlink:to="CostOfSales" order="2" weight="-1" use="optional"/> The example shows that there are defined two calculation arcs providing details concerning relations between Gross profit, Revenue and Cost of Sales. In Income Statements, Gross profit is the difference between the other two. Therefore, we assign weight attribute value to "1" on the arc connecting Gross profit and Revenue and "-1" between Gross profit and Cost of Sales. The reason why there is a difference between calculation and presentation linkbases, is that the total element that stands for the summation of all others usually appears at the bottom in the financial statements whereas in the calculation linkbase it must be placed as the top concept. Presentation