In a long campaign, with extensive ground unit use, you end up with many units that are understrength.
Making replacements to bring them back up, can be quite a bit fiddly at times, due to the complexity involved, and the different ratios of what needs replaced.
So here is what I suggest: An understrength unit, can have the "use for replacements" checkbox ticked.
Ground unit construction, will use elements from any unit set as a replacement, to reduce the time&cost of units under recruitment.
E.g. I bring back an understrength Tank Battalion to Earth. They have 40 out of 90 tanks they should have at full strength. I set that formation to "use for replacements"
I recruit a new tank battalion on Earth. Instead of paying the cost of 90 tanks, they use the 40 from the understrength unit, and so the cost is for 50 tanks.
This would help the player, by providing a straightforward way to consolidate remnant units.