IBM MQ and IBM MQ Appliance MQOPEN call

When a MQ Application wants to connect to an MQ Queue Manager, an MQCONN (connect) is issued.
If it succeeds, an MQOPEN call can be issued to connect to an object (e.g., a Queue).

Title: MQOPEN - Open object
https://www.ibm.com/support/knowledgecenter/SSFKSJ_9.1.0/com.ibm.mq.ref.dev.doc/q101870_.htm

in this case do we say MQ Application is a client?

Typically an application opening a connection to the queue manager, see if diagram helps to visualise

https://www.ibm.com/support/knowledgecenter/prodconn_1.0.0/com.ibm.scenarios.wmqwas101v9.doc/images/logical_topology_addwas.gif

Original article - https://www.ibm.com/support/knowledgecenter/en/prodconn_1.0.0/com.ibm.scenarios.wmqwas101v9.doc/topics/ts_final.htm?cp=SSFKSJ_7.5.0

> in this case do we say MQ Application is a client?

Yes, even if it is on the same machine and using a "bindings connection" to the Queue Manager (QM).
Bindings connections use shared memory, instead of the network, so there are some differences (e.g., no TLS/SSL), but they are likely to be much faster.