You can now filter Landsat products by Landsat tiers, which determine quality and level of processing of products. There are three tiers - Real Time, Tier 1 or Tier 2. Real Time is a temporary tier for the Landsat 7 ETM+ and Landsat 8 Level 1 collections. Products stay in this tier for a day or two until the final tier is determined, after which they are deleted and replaced with the T1 or T2 version. Tier 1 is for products of a higher quality, while the rest are categorized into Tier 2. More about tiers here.
For collections with Real Time tier, you can request Tier 1, both Tier 1 and Real-Time, or all tiers. For other collections, you can request Tier 1 or all tiers. This way, you can omit either Real Time or Tier 2 products. The default for all collections is all tiers. Filter is documented in all collections under Filtering options > tiers.