Refeeding questions

Hi,

Undoubtedly a dumb question, but sometimes after NG feeds (via pump or gravity) when I check on baby, they have digested milk back up in their tube. I get confused on if this is considered "residual" or what it would be called (reflux?), and whether I should refeed it if it's quite a bit, especially if it's right before their next feed (wondering if it will just make it worse on next feed). 
There are some questions I don't feel the safest asking as my unit is not very welcoming and this is something that didn't come up in orientation.

Thanks!