The 29 AP and the 42 HEAT are a numeric indication of the penetration potential of the round. To determine the degree of penetration multiply the number by 15. There fore the AP round can penetrate 435mm of steel where the HEAT round can penetrate 630mm of steel. The "6 Area" is similar and is a measure of the output of the small arms value of the weapon. The "52 total ammo" is the basic ammo load for that weapon.
I recommend you review "The Poor Man's Mod Guide 4" that discusses where these number come from. Also, section 6.6 of the User's Manual gives the definitions for the codes used in the subunit inspector.
The information in the SUI is a bit cramped and cryptic. We are planning to expand the SUI in Southern Storm and onward to include the information in real world terms and with more explanations of platform capabilities. This will better help new player and also be of more interest to the Grogs out there.
Feel free to ask any questions. That's why we are here.