I understand that in the case of Apple Mail that the tags applied to those email are local. Does that mean the tags will not show up on my 2nd Mac and iOS devices?
Tags will only automatically show on other eM Client installations if they are Server side tags.
To see if your server is supported "when you setup Tags via “Menu / Tags” (Pc) or “Tools / Tags” (Mac), “click the dropdown arrow on the right of Local” at the top left of the Tags section and select your server.
Now “if there is only Local tags available in the dropdown” then that means your server is not supported and “you will have to save / export your Local tags” to your hard-disk, and then import those on your other eM Client desktop or laptop.
You can export Local Tags via “Menu / File / Export / Export Settings to XML”. Then click Next and “just select / check Tags” and select your destination folder to save. Then click Finish.
Then manually “copy the saved Settings.xml file across to your other desktop or laptop computer eM Client installations” and "import that XML file via “Menu / File / Import / eM Client Settings”. Then click next and select your saved Settings.xml file and click Finish.
To get the Local Tags from eM Client for desktop to your eM Client iOS mobile app, go in the iOS app to “Menu / Settings / Import / Export” and choose either “Import Settings from file” if you have also manually copied that saves desktop Settings.XML file to your Apple iOS device in a folder via a computer USB cable or via Wifi.
Or you can choose to directly “Import via the QR Code” option from the desktop to the mobile app.
To export the Local Tags via the desktop directly to the mobile app, go to “Menu / Tools / QR Export” (Pc) or “Tools / QR Export” (Mac) and “just select the Tags in the export options”. Then after selecting the Tags, at the end of the export wizard, the Barcode will appear for you to scan.
Then open the iOS mobile app and go to “Menu / Settings / Import / Export” and choose “Import via QR Code” and follow the prompts to import. The iOS app will then have the same Local Tags.