Questions on Transfer Learning from ModelZoo

Question 1: As part of a project I trained a custom model from the tensorflow model Zoo found here:

https://github.com/tensorflow/models/blob/master/research/object_detection/g3doc/tf2_detection_zoo.md

Now I retrained this model from a checkpoint, so that it would look specifically at one category from the coco dataset. However, I’m wondering if the training was even needed, seeing as the model was initially trained on the coco dataset. So my question is, does retraining on the same dataset have advantages, when looking at one particular element of the dataset? (narrowing from 90 to 1 categories).

Question 2: To remedy this, I thought I might want to train a model from ‘scratch’. They provide a link in the above link to some untrained model pre-sets.

https://github.com/tensorflow/models/tree/master/research/object_detection/configs/tf2

However I noticed in the config files they link, it still says:

Trained on COCO17, initialized from Imagenet classification checkpoint

and the model config has an entry:

fine_tune_checkpoint: “PATH_TO_BE_CONFIGURED/mobilenet_v2.ckpt-1”

Can anyone explain what’s going on here and how a model can be trained ‘from scratch?’