What they probably mean is that they have a transaction that consumes $1000000 on their address and creates two outputs: $100 withdrawal for client A, and $999900 on a "change" output. Later withdrawals have to link to that change output (bitcoin is a directed acyclic graph of inputs-outputs). If that transaction is not confirmed, then later txs also cannot be confirmed and backlog grows.
What they probably mean is that they have a transaction that consumes $1000000 on their address and creates two outputs: $100 withdrawal for client A, and $999900 on a "change" output. Later withdrawals have to link to that change output (bitcoin is a directed acyclic graph of inputs-outputs). If that transaction is not confirmed, then later txs also cannot be confirmed and backlog grows.