Aggregation status means the total cost of ALL day trades added up together in one day cannot exceed your Starting Day Trade Buying Power.
A Pattern Day Trader (PDT) account begins the day with margin equity (ME) of $1,500 and starting day trading buying power of $1,500. The account has a prior, open, not yet past due, DT call.
Trade 1 (9:00 AM)- BUY 50 ZZZ $55 ($2,750)
Trade 2 (10:15 AM)- SELL 50 ZZZ $56 (Option BP increases to $3,050)
Trade 3 (11:00 AM)- BUY 100 QQQ $29 ($2,900)
Trade 4 (1:15 PM)- SELL 100 QQQ $29
While neither trade individually exceeded starting day trading buying power, the fact that this account is in aggregation means that you must total up all day trade requirements. The total of the two day trades is $2,750 + $2,900 = $5,650. This amount exceeds the starting day trading buying power by $4,150, and a DTM call will be issued for this amount.