What is the relation between transaction OBBS and OB08
T-Code OBBS is used to maintain the ratios of translation for the conversion of the currency and the rates of exchange are maintained in OB08. In other words in OBBS( Maintain translation ratio) you define/ maintain the relation between any two currency and the Exchange Rate is maintained in OB08
OBBS is a pre -requisite to OB08.
For e.g suppose If the ex.rate is 1 INR = 1 USD you first need an entry in OBBS.
If the OBBS entry defines the relation INR / USD is 1:1 than the mandatory OB08 entry is INR / USD is 1:1 (for a given from-date).
Imagine if the OBBS entry INR / USD is 10:1 than your OB08 entry is INR / USD is 10/1. The OB08 entry is always depending on an OBBS entry.