Explanation:
Box 1: Hides the control in the body of the form.
The Client API form context (formContext) provides a reference to the form or to an item on the form, such as, a quick view control or a row in an editable grid, against which the current code is executed.
setVisible sets a value that indicates whether the control is visible.
Box 2: Add an event handler to enable a function named TestFunction to run when the business process flow stage changes.
addOnStageChange adds a function as an event handler for the OnStageChange event so that it will be called when the business process flow stage changes.
Reference:
https://docs.microsoft.com/en-us/powerapps/developer/model-driven-apps/clientapi/clientapi-form-context
https://docs.microsoft.com/en-us/powerapps/developer/model-driven-apps/clientapi/reference/formcontext- data-process/eventhandlers/addonstagechange
https://docs.microsoft.com/en-us/powerapps/developer/model-driven-apps/clientapi/reference/formcontext- data-process/eventhandlers/addonstagechange

Explanation:

Explanation:

Box 1: PreValidation
For the initial operation, this stage will occur before the main system operation.
This provides an opportunity to include logic to cancel the operation before the database transaction.
Box 2: PreOperation
Occurs before the main system operation and within the database transaction.
If you want to change any values for an entity included in the message, you should do it here.
Avoid cancelling an operation here. Canceling will trigger a rollback of the transaction and have significant performance impact.
Box 3: PostOperation
Occurs after the main system operation and within the database transaction.
Use this stage to modify any properties of the message before it is returned to the caller.
Reference:
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/event-framework

Explanation:

Entity: LogicalName
Querying the EntityMetadata entity type:
GET [Organization URI]/api/data/v9.0/EntityDefinitions(LogicalName='account') Attribute: LogicalName Retrieving attributes:
GET [Organization URI]/api/data/v9.0/EntityDefinitions(LogicalName='account')/Attributes( Relationship: SchemaName Querying relationship metadata:
Entity relationships can also be queried using the RelationshipDefinitions entity set. You can use a query like the following to get the SchemaName property for every relationship.
GET [Organization URI]/api/data/v9.0/RelationshipDefinitions?$select=SchemaName Global Option Set: Name Querying Global OptionSets:
GET /api/data/v9.0/GlobalOptionSetDefinitions(Name='metric_goaltype')
Note: Retrieving items by name is generally easier because you probably already have some reference to the metadata item name in your code. The following table lists the alternate key properties for retrieving metadata items by name.

Reference:
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/webapi/query-metadata-web-api