a few questions about the db table

[guiz]

I have read the table with phpMyAdmin and I have a few questions about the entry.

in the table pixelpost_categories I have 3 fields: id, name and alt_name. in the field alt_name I have the entry "default" or a blank field. can you tell my why?

in the table pixelpost_pixelpost in the field category I have an entry (number) or a blank field.
and what is if I assign more than 1 categories to an image?

pixelpost works fine, I don't have errors, but I asked me, why I have these blank fields.

[GeoS]

1. There is set string called 'default' for alt_name as default value when in insert it is empty.

2. There is no difference and categories for each image are stored in separate table (*_catassoc). Vaule in *_pixelpost table is only for backward capability.