How to translate from data sheet to actual pin function (blank Atmel SAMD21)
It is not unusual to see multifunction I/O pins on microcontrollers, so the tables in the datasheet (particularly table 7.1) identify the alternate functions associated with each pin (and by extension, the peripheral they can attach to internally).

This part is not much different from many variants using ARM Mx cores with multiple peripheral mappings.

The default pin function for all standard I/O pins is an ordinary digital I/O port (See the copied text from the datasheet at the bottom).

As an example, I will take a couple of lines from the table in question:



On the left is the physical pin (if it exists - depends on how many pins the package has and controller variant).

The next part is the port bit attached to the pin itself.

To the right of here is the supply pin(s) that actually power this I/O bit.

In the next column, we see I2C - this is explained in note 3:


  
  Only some pins can be used in SERCOM I2C mode. See the Type column for using a SERCOM pin in I2C mode. Refer to Electrical
  Characteristics for details on the I2C pin characteristics.
  


This tells me that I can only use pins with I2C against them for the serial control unit(s) to be able to use an I2C interface.

To the right are alternate functions (if any exist) of A to H. 

PB13 for example, has alternate functions in

A: EXTINT[13] which attaches to the interrupt controller if selected. 

B -> X(11) for the touch controller

C -> SERCOM (serial module controller) number four which will attach to PAD1 as documented in the SERCOM chapter in the datasheet.

D, F and G have no alternate function.

E and F permit the pin to operate as part of the Timer / Counter module

Function G attaches the pin to the I2S (audio) module.

Function H attaches the pin as a generic clock (See the clock description in section 14 for details).

To use any function, you must (at least)


Turn on the clock associated with the function (or any writes to the peripheral are ignored - see the power manager module)
Enable the specific peripheral function
Programme the PMUX register(s) appropriately (see section 23.7).
Enable the multiplexed function in PINCFGy by setting the PMUXEN bit for the specific pin.
The other bits in PINCFGy must also be appropriately set.


Without knowing the specific setup you need, I cannot give further guidance.


  Bit 0 – PMUXEN: Peripheral Multiplexer Enable
  This bit enables or disables the peripheral multiplexer selection set in the Peripheral Multiplexing register
  (PMUXn) to enable or disable alternative peripheral control over an I/O pin direction and output drive
  value.
  Writing a zero to this bit allows the PORT to control the pad direction via the Data Direction register (DIR) and output drive value via the Data Output Value register (OUT). The peripheral multiplexer value in PMUXn is ignored. Writing '1' to this bit enables the peripheral selection in PMUXn to control the pad. In
  this configuration, the physical pin state may still be read from the Data Input Value register (IN) if PINCFGy.INEN is set.