When constructing a BattleMech or Combat Vehicle the Tech Rating is based off the highest rated item.
If you were to have a Mongoose MON-66 during 3025 play with its rating of E it would be subject to the 5.0 modifier for spare parts cost.
When using infantry is the Tech Rating based off the highest rated item or just the tech rating of the unit itself.
Jump Infantry are normally a tech D unit. Some weapons used by infantry such as those I posted above have a Tech Rating of E.
Technology Rating
In the Infantry Weapons and Equipment Tables used for
creating conventional infantry, all items receive a Technology
Rating in addition to a tech base. This rating (expressed as a
series of letter grades that define the level of technology and
its availability through the major eras of Classic BattleTech
history) helps to define the item’s level of sophistication and
may be used to standardize the level of advancement the
unit demonstrates for purposes of era-based campaigning.
Beyond this, however, an item’s Tech Rating has no bearing
on conventional infantry design. See Costs and Availability (p.
274) for more on Tech Rating.
So you have the Mongoose at tech E
You have an Jump Platoon(Rifle) at tech D
You have a Jump Platoon(Support PPC) at tech E
The rules are clear about the BattleMech, between 2860 and 3040 the parts for the Mongoose are extinct or just coming back into circulation so the cost modifier for parts would be 5. The infantry unit however does not contain anything that has gone extinct. Even though the Support PPC has a tech rating of E it never goes extinct. Based on that I would think infantry would be based on the motive rating in this instance rather than the overall tech rating.
I hope this clears this up a bit.
