Just in case one co. is acting as a securities broker for another, margin a/c should come into the equation.

Exact Globe is a software program. The dekkingsrekening in Exact Globe is defined in an Exact Globe users' forum as follows:
Waarderen: ... Het verschil wordt op rekening prijsverschillen gezet (***verschil tussen verkoopboeking en inkoopboeking) en dat kan dan worden gebruikt als dekkingsrekening.***
See http://www.exactforum.nl/messages/37/585.html

If we now search for the most commonly used term to describe the valuation of the difference between selling price and purchase price, we find that this is defined most commonly either as capital gain, or profit. Since we do not know the exact type of item which is being valued, the most neutral overall applicable term would be simply "gain".

However, the plain meaning of "dekking" in Dutch is "cover". It is therefore my hunch, although I cannot substantiate this with a reference from the Internet, is that a "dekkingsrekening" is a calculation of the minimum price at which something can be sold which will cover for the cost price and various factors such as exchange rate variation, depreciation, etc. so that the seller will be "covered" for these factors and still sell at a profit.