Transport database: understand, size and troubleshoot

hi ChristopheCLDC !

When I run 

[xml]$bp=Get-ExchangeDiagnosticInfo -Process EdgeTransport -Component ResourceThrottling; $bp.Diagnostics.Components.ResourceThrottling.ResourceTracker.ResourceMeter |select resource,currentresourceuse,pressure

I get:

Resource                                       CurrentResourceUse Pressure
--------                                       ------------------ --------
PrivateBytes                                   Low                5       
SystemMemory                                   Low                36      
UsedVersionBuckets[G:\Queue\mail.que]          Low                1       
JetSessions[G:\Queue\mail.que]                 Low                13      
CheckpointDepth[G:\Queue\mail.que]             Low                0       
DatabaseUsedSpace[G:\Queue]                    Low                5       
UsedDiskSpace[G:\Queue]                        Low                5       
UsedDiskSpace[E:\Exchange\TransportRoles\data] Low                51      

so I am curious what does the "JetSessions" resource mean? 

I haven't found any referencies in the microsoft article about backpressure(

In this article: https://learn.microsoft.com/en-us/exchange/mail-flow/back-pressure?view=exchserver-2019