What is stored in the registers of the PIC18 controller?

Question

Programming

PIC18F84A controller.
Initialization of the PORT and TRIS registers.

TRISA = %11111111 //все выводы порта А - входы
PORTA = %00000000 
TRISB = %00000000 //все выводы порта B - выходы
PORTB = %00000000

It is not clear about PORTA = %00000000 - on Habré I read that these are "all outputs of port A with a low level." What does it mean?
It is also not very clear what the parameters PORTA.0, PORTA.1, PORTA.2, etc. mean?
for example

if(PORTA.0 == 0)
{
...
}

Answer 1

it is "all port A pins low". What does it mean?

This means that the pins associated with port A will go low. But only if the corresponding bit is configured as an output. A pin configured as an input is not affected by what is written to port A.
These are individual bits of Port A.

Answer 2

Some strange notation, usually the Microchip has binary numbers written as 0b00000000. And on PIC18 it's better to use LATA rather than PORTA to avoid "read-modify-write" problem (use PORT to read port status, and LAT to write logic level).