A subline imprint is a partial re-branding of a franchise- typically with a sub-title reflecting a theme or gimmick running through the entire franchise.
Examples include Battle for the Spark during Beast machines and AllSpark Power for the 2007 movie. The second season of the Beast Machines was re-titled to feature the subline imprint, while the Allspark Power toys occupy a loose umbrella story which occurs after the events of the movie.
Purchasing agents known as "buyers" make purchasing decisions for hudnreds of American Retail stores, making their best guess what products will sell and how many while trying to avoid being stuck with the dreaded 'old product' (unsold toys) when the toyline ends.
Buyers purchase many toys from early waves reasoning that even if the toyline flops these will sell eventually over the line's 18 month 'lifetime.' Toys in later waves (8-ish,) represent a higher risk because they have only a 6 month window to sell, and tend not to be purchased as heavily. as a result toy companies spend less money developing later waves, often heavily featuring repaints and remolds, making them even less attractive to retailers.
For their part Consumers try to avoid purchasing the pegwarmers and shelfwarmers left over as a toyline winds down, but lacking the encyclopedic product knowledge known only to Buyers, fans and small children, they learn to suspiciously avoid any packaging they recognize from the last birthday/Christmas/Kwaanza etc. (Again making later-wave toys less attractive to retailers.)
A subline imprint refreshes the packaging for consumers (orange may become bright green in the same layout) while assuring the Regional Buyers that "No, seriously, this is a new toyline, you should purchase it more heavily!" Buyers aren't particularly fooled by this argument, but the re-branding is understood to be a sign toy companies has invested in these tail-end of these waves rather than filled it with junk, making a larger Retailer purchases a safer investment.