Select for update nowait

Suppose steps below:
1. begin TX1
2. in TX1, do
update table1 set a = 1;
3. begin TX2
4. in TX2, do
select * from table1 for update nowait;

Since TX1 is holding a lock, so TX2 failed to acquire a lock. Then the "select * from table1 for update nowait" will return with an error. I called it "noblocking mode"

On the other side, if in step 4 do
select * from table1 for update
instead. the TX2 will keep waiting until TX1 committed or rollbacked, then acquire the lock, then return successfully. I called it "blocking mode"