What (in simple terms) is the meaning of read/write semantics and of object semantics? Why is the following statement true?
Debit/credit write to bank accounts and are defined to conflict under read/write semantics whereas under object semantics they do not.
Without more context, I'm not sure if this answers the question.
Section 2.1-2 in the below PDF provides an overview.
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.48.4184&rep=rep1&type=pdf
Here are some key points.
Read/Write
"The two phase locking protocol for read/write model maintains two types of lock for each object, namely read lock and write lock. A transaction must obtaion a read(write) lock before it can read(write) a data object. Two locks from different transactions conflict if both locks are applied on the same data object and at least one of them is a write lock. A transaction cannot obtain a lock if the lock conflicts with any other lock on that data object."
So you cannot read if another is writing or write if another is reading. But you can read if another is reading.
Object
"An object maintains a state and provides a set of operation that act as the sole means for transactions to access and modify the state of the object. ... Two commutative operations can be executed concurrently even when both the operations are update operations."
Now you can allow reads/writes for things where the order of operations are not of consequence.
They go on to explain how a deposit and withdraw will not commute(withdraw could be denied for insufficient funds), but two deposits can commute. These two deposits would block each other and have to occur in sequence under read/write but under object semantics they can occur concurrently.