Oracle define this feature as:
A segment in the chart of accounts can be designated as a 
secondary tracking segment, in addition to the balancing 
segment, to perform more detailed analysis within Oracle 
General Ledger. The secondary tracking segment is used in 
the revaluation, translation, and fiscal year-end close 
processes. The system will automatically maintain 
unrealized gain/loss, retained earnings, and cumulative 
translation adjustments by unique pairs of balancing 
segment and secondary tracking segment values.”

Therefore, you can use any segment, except the balancing 
segment or natural account segment, can be specified as the 
secondary tracking segment.