Unfortunately there are no standardized industry specifications or measurements. However, the following rule-of-thumb applies.
Scoop neckline with relaxed sleeves and straight relaxed sideseamed body.
Feminine neckline, longer cap sleeve, contoured and side seamed body with a looser fit.
The slim fit T-shirt has a narrow cut that focuses on minimal fabric, therefore, leading to a tighter fit that looks more modernized and hugs tightly to the body.